Yeah, let me just look up what kind of space low-quality [128kps] audio files, Vocal, would cost...
OH LOOK AT THAT
Even for all the text in R/B/Y, you're looking at massive amounts of data, which could be spent on... I'dunno, 3D environments, full-motion battles, 3D moves, improved item and HM animations, and generally improving the game.
Voice acting can make or break a game. In this case, it would shatter Black and White.
You can't manipulate voices as easily, and frankly it would lose a lot of what the *games* are about. Remember, the anime is an adaption of the games, not the other way around.
I think it might come in handy in-case their is a wired name the say and you dont know what it is and for people who dont like it they could turn it off in the options.