This has been discussed before, just looking at it from a financially perspective the amount I would want to be a Joint Sponsor would be more than you need to show in assets, just look at the potential downside.
I watch well now often listen to Hackings shows, he had a case on recently where there seemed to be an issue involving a Joint Sponsor and Fraud, now it did not make much sense to me, implied it arose out of a non traditional relationship with the Joint Sponsor but where is that a requirement?