Movies are predominantly a visual medium, and movie stars gain their tawdry spot on the retail shelf in large part because of their faces. Sure, they have personalities and acting skills and voices, too, and often bodies we're lucky enough to see without clothes. But a movie star can make do without these things. They absolutely cannot become a star without a face we can bear to look upon.
So this boom of stuffing animated cartoons (or, like Marmaduke, the latter half of a live-action/CGI hybrid) with movie stars is more than a bit idiotic -- tell me you didn't double-take at that first Shrek poster, where "Mike Myers" was proudly credited beside a head-shot of someone or something green that definitely was not Mike Myers. The ads for Rango trumpet JOHNNY DEPP - over a CGI chameleon. When I see a movie starring Owen Wilson, I want the broken nose and dishonest smirk, not a CGI animal. Jim Carrey starring as Horton the Elephant isn't really anything but Jim Carrey picking up an easy check. We should demand more. A week in the recording studio doesn't equal "starring" in a $100 million movie. They should be honest and just call it radio.
Especially if they have indistinct voices, like Myers. But donating voices is nothing new, nor has it ever before been a big deal. Phil Harris and Peggy Lee and whatever kid did the voice for Thumper never saw their names over a Disney title. Hollywood history is littered with actors who have overdubbed each other and got no credit at all - Audrey Hepburn and Deborah Kerr karaoked to Marni Nixon's singing, but no one cared. For Greystoke, an uncredited Glenn Close dubbed Andie McDowell's entire role, presumably because McDowell couldn't shed her South Carolina twang. Did you ever hear anyone say that Close was "in" the movie? The ever-sensible James Earl Jones didn't want billing for doing Darth Vader's voice in the first Star Wars, and he didn't get it.
So, honestly, Jack Black isn't really "in" Kung Fu Panda, nor is Brad Pitt "in" Megamind. Charlotte's Web wasn't a Julia Roberts movie, and so on. Had unknowns read the dialogue for these movies, the end result would be more or less the same. The movies might even be better. In most cases, it seems that the "presence" of stars in animated fluff aimed at kids acts as an anesthetic for the parents, who are probably just terrified of being stuck with a "squeakquel" or talking guinea pigs. They are the most beleaguered class of filmgoer, and will fall sucker for any bait. The rest of us have no alibi.