Celebrity voice actors are for adult bonus. And it's certainly not new. We only need to look as far back as Louis Prima as King Louie in The Jungle Book. The character was named after him. Not to mention even further back, Ed Wynne as the Mad Hatter. Not to mention how many celebrities revitalized their careers as voice actors. Paul Reubens, Mark Hamil, Deidrich Bader, Gilbert Gotfried... even Weird Al Yankovic has found a nice in between albums gig as a voice actor, playing either himself (as on Batman: Brave and the Bold, among others) or as a character (The Learnerer on Word Girl, Wreck-Gar on Transformers Animated). I do prefer the classically trained voice actors, but I understand the need for celebrity voice acting. And when it works, it's actually pretty good. If not, you get Dreamworks' Sinbad, starring people with not too remarkable voices. I like how Pixar has the guts to have lower celebrities as voice actors in every so many movies. I mean, Up's headliners were Christopher Plumber and Ed Asner. Heck, that's another one. Ed Asner does a lot of cartoon voice acting (we all remember Cosgrove from Freakazoid, right?).
Personally, I miss Brad Garret as a voice actor. There are celebrities with voices that are perfectly cartoony. Wayne Knight was a great cartoon voice over as well. I love Dojo the Dragon, Igor, and especially his superior version of Evil Emperor Zurg. And Patrick Warburton. I think I like him better as Buzz Lightyear than Tim Allen.