I don't think it has anything to do with Jon thinking he's too good to sing Richies stuff. I think it's more about leaving Richies solo work to Richie, not invading his space, or taking away from from Richie in any way.
The same way JOn didn't do the red carpet on the premiere night of Memphis last year but Richie and Tico did. It was David's night and Jon didn't want to distract from or diminish it in any way for David, so he stayed away. Do you really think he was any less proud or less excited for David than any of the others??