Yeah, it does because at the core of voice acting is acting. Any accomplished actor would be able to be a good voice actor as well. Yes some people might have a difficult time expressing emotion without motion but anyone who is an accomplished actor already has the qualification to be a voice actor... acting experience.
The question is, will these younger siblings of Disney stars be good actors? Maybe, maybe not but you should give them the benefit of the doubt. They didn't get shoe-horned in there for publicity they were chosen by the directors. Any publicity made from it is merely a bonus but Disney has always used "non-anime" actors for their Ghibli dubs and I'd expect nothing less cause Disney can pay more and thus actors who usually don't do VA work would be interested and audition for the roles.
rust, did you boycott Kingdom Hearts because Mandy Moore was Arieth and Lance Bass of all people was Sepharoth? Darn good game you missed out on if you did.
Didn't live up to their full potential? What exactly IS that? If we're talking Dreamworks, then yeah they throw every star they can get into movies and it doesn't really make them good but Pixar's casting has always been superb. Did Jason Lee keep The Incredibles from reaching it's potential? Of course not, he was one of the best parts of the movie as that character. He brought that character to life. Or perhaps you mean something more like having Sam Jackson as Frozone. I loved him in that role and he was perfect for it. How about Billy Crystal and Jon Goodman in Monster's Inc? They were fantastic. Even Owen Wilson as Lightning McQueen was quite good. Tom Hanks and Tim Allen in Toy Story? A very endearing pair. I just don't see what you're getting at here. What potential was lost by having accomplished and perhaps popular (at the time) actors in the main roles or even bit roles of these animated features? They didn't sell the movies, the movies sold themselves and they helped enhance the experience with their excellent work.
And so what if anyone was shoe-horned into the cast for this Miyazaki Movie, that's par for the course with the Ghibli releases but all the dubs have been wonderful. Is it so bad if they use that to get less cultured people to watch the movie? Seems like a good way to get more people to see it.
Or hey let's look at Japan where its COMMON for pop stars to voice in animated projects. Is that okay... because they're Japanese? What's the difference? There is none. Just go watch the dang movie if you had any interest in it in the first place.