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Well, what happens if you go, and they want to ask her questions, but she's at work?  It's really not that complicated to work out why it is important that your spouse attends.

Yes the spouse must attend, even if the letter doesn't say it.
 
She may never be called in, but the chances are high if I-751 is pending and you're naturalizing under 3 year rule.

You are right, it is not obvious, but rather intuitive with both an ROC and N400 pending.  That being said, it is possible that even with both pending, the IO may not even have the I751 package available, and may not require your wife there at all.  It is a tough conundrum, but I tend to err on the side of caution, and maybe buy your wife some flowers   in case the IO doesn't deal with the I751 and your wife feels like she wasted her time.
 
Good Luck!
 

Ask her to go. Odds are high that they will want to do the 751 interview at the same time. 
If she doesn't show, then you get put back in the queue and are waiting who knows how long.

My husband's N-400 interview letter said nothing about his pending I-751 or a combo interview. There was nothing  saying I was required to be there. We knew there was a chance it would be a combo interview, so I went along. To be honest, I felt it was important for me to be there, to support him on what could be a huge event in his life. It did turn out to be a combo interview and he took the oath that afternoon! It would have been a mistake for me to not have attended.  Since we were both there, it was easy for us to go out, when it was over, and celebrate!

Sometimes they ( uscis) sends an email or letter shortly before just to remind you about bringing your spouse.

I just had my interview less than a month ago, and my spouse went with me.
I received two notifications, the first letter was about my naturalization form N-400 and it did not say that my spouse was necessary to be present.
But, the second notification came saying that I was having a combo interview, and the spouse should be present if we were still married or if not, show proof of divorce (the 797 notice had a printed sheet saying about the combo interview, uncomfortable I don't have this document because stayed with the IO, and I was excited about being approved and didn't notice that).
The officer started saying that he was conducting a combo interview, the first was the ROC and my spouse could answer any of his questions, but during the second interview N-400 he should stay quiet and only I could answer.
About documents that he asked for and were not in the letter listing: my spouse's birth certificate and deed/mortgage documents showing that we both owned our home. 
IMO: Your wife should go to the interview to support you, is the last phase that you have to do on your immigration process, also as part of the ROC requirement is her be present to them do any questions if necessary. Otherwise, is a red flag to the immigration officer. 
 

It is interesting how different offices or different IO's handle the combo interview.  In our case we were in the same situation, but did not receive any notice of a combo interview.  Regardless, I went with my wife for her N400 interview, and when the IO came out to get her, she requested that I also come back for an I751 interview.  That interview lasted about 5 minutes where we verified the I751 form, added a more recent tax filing transcript, and then I was escorted back to the waiting area before the IO conducted my wife's N400 interview.

As I always told my troops when I was active-duty U.S Air Force "Over prepared is fully prepared".  If in doubt, spouse should attend. In my case, local office notified us 2 days prior that I was required to attend along with wife for combo interview. 

Well I wouldn’t say it’s obvious as nowhere states she does. 

False alarm everyone we found the old one! Thank you for your time and advice! 

False alarm everyone we found the old one! Thank you for your time and advice! 

She lost it along time ago and we have ordered a new one but it’s saying 6-8 weeks processing before it arrives and that was 2 weeks ago my interview is in 15 days. 

He is incorrect and it would be rejected if more than 90 days prior to expiry of your 2 year card....unless it is a waiver (divorce/abuse) filing, which can be submitted & will be accepted before or after the usual 90 day window without explanation or penalty

Don't send it prior the 90 Days window, I sent my I-751 5 days earlier from the 90 days because I got scare that due COVID the shipping process would take longer, they received in two days after I send it, they rejected all and send it back this delayed my process almost 2 months.
 
Use this filling calculation to know the exact date:
https://www.uscis.gov/forms/uscis-early-filing-calculator

My cousin package arrived one day before of 90 days and it was rejected. Then he has to submitted again 

I see that someone has a date calculator in this thread.
Personally I say don't try to be smart to get exactly 90 days. Drop it in the mail at the earliest 89 days else you will up having to resubmit.
 
I don't know which time zone USCIS uses. Also if they use the postmarked date or the date of arrival.
 
89 days folks. That's when I will drop it in the mail.

If you have a 2 year conditional green card you file an i751 to remove the conditions within 90 days of the expiration of the 2 year card. You get an extension letter for 2 years, most ROC LPR cases are approved within the two years and you get a 10 year green card after ROC approval. 
 
You can travel, get a job, or renew a driver's license with the old expired conditional green card together with the expired green card. 
When you apply for an i751 your LPR status does not expire, only proof of your continued LPR status expires. Only an immgration judge takes away your LPR status.

As long as you are still married you can file for naturalization at 3 year mark with an open I-751 case. 

Yes, as long as you remain married to a US citizen spouse for the whole 3 years since getting your green card. That is our case, we have both a ROC and naturalization pending at this time. Our n400 is scheduled for an interview on May 16th. Hopefully they will adjudicate our ROC case at the same appointment as we have both cases pending.

No, your application will be rejected, you can only apply a joint i751 application WITHIN 90 days before your conditional green card expires. If you check the i751 instructions you will see when you can file an i751 application https://www.uscis.gov/i-751.
 
In fact you MUST file within the 90 day window otherwise you will be late or too early. 
 
Between now and when you can file you can collect bona fide marriage proof to submit with your i751 application. Joint Tax Return transcripts for each year of marriage, joint lease, copies of state ID with same address (driver's licenses), w2 or 1099 docs for each year married show common address, joint bills, joint bank statement for each month of marriage, birth record of children together, photos together, proof of travel together, even copy of mail sent common address, and so on.
 
A i751 waiver case can be filed at any time, but the joint l751 must be filed within 90 days of CGC expiration.

Sending 6 months early does not apply to removal of conditions.  As others have said, the very earliest USCIS can receive a joint I-751 package is 90 days prior to expiration of the 2 year conditional card.  If CBP told you something different, ignore it.  
Note : I would not send the I-751 package until a couple days after the 90 day window opens.

Well he doesn’t know anything about USCIS policies/immigration law. You can send it earlier but your application will be rejected and sent back to you. 

It's recommended to pay the USCIS Immigrant Fee after getting the visa: https://my.uscis.gov/uscis-immigrant-fee
 
Your endorsed Immigrant Visa is a I-551 document while you wait for the Green Card. You may be able to work while waiting for SSN:
 
https://www.twc.texas.gov/news/efte/employees_without_ssns.html "It is certainly legal to hire someone who is authorized to work in this country, but who does not have a social security number or who chooses not to disclose it."
 
For employers that use E-Verify, the employer can select "Awaiting Social Security number." https://www.e-verify.gov/faq/my-employee-applied-for-a-social-security-number-ssn-but-has-not-yet-received-it-what-should-i "If a newly hired employee has applied for, but has not yet received his or her SSN (e.g., the employee is a newly arrived immigrant), attach an explanation to the employee’s Form I-9 and set it aside. Then create a case in E-Verify using the employee’s SSN as soon as it is available. If you are unable to create a case by the third business day after the employee’s first day of employment, select Awaiting Social Security number when prompted by E-Verify."

You SHOULD receive it automatically.  In the event it doesn't arrive, you will HAVE to apply for it in a few months.