I think celebs work at WM when they have someone sing "America The Beautiful", do backstage interviews, or be someone's manager (like Trump was at WM22). Also, look at Wrestlemanias 1 and 2 (I have the WM anthology of the first 20 WMs), and in the first two, they had heaps of celebrities, but aside from Mr T, none wrestled. They used major names as ring announcers, referees, commentators, bell-boys and managers, and it worked. (WM1 contains such celebs as Mr T, Liberace, Muhammad Ali and Cyndi Lauper, and they all filled their roles perfectly).
I don't like them wrestling though, as it makes it look like "anyone can wrestle". What if Cena had FU'd K-Fed, and K-Fed hadn't braced himself properly (in wrestling, the person having the move done of them does 90% of the work) and got injured, and then sued WWE. I would prefer them to stay away from wrestling in the ring (unless it is Dwayne Johnson).