If The Undertaker came back and demanded to go into WrestleMania as the WWE Champion and barely did any work on Raw or SmackDown in the months between the show, I'd have a problem with it. The Undertaker knows his limits. He knows he can have that one match and appear on Raw and SmackDown for a few promos. That's why he doesn't get criticized as heavily as The Rock does.
Let's face it, The Rock doesn't have to be the WWE Champion to draw ratings. If his match with Cena at WrestleMania is so 'big' why add a title to it?