I don't think the letter means anything. It just has a placebo effect. USCIS know that you have a conditional GC and that you are waiting for the ROC. I sent my N 400 2 months ago and I did not include a letter. I added the GC and the extension letter. I think it is sufficient. I associate the combo letter as a begging.
And in the most honest way, you don't want another I 751 interview, you d want it waived !