I'm sure Puchimasu was picked as a way to test the market. The episodes are short, which probably made the rights cheap to pick up. We don't know if they nessecarily bought the rights to the whole franchise yet, guys. Maybe they just got Puchimasu because it was cheap. Second, half of the cast doesn't require voice acting short of Pokemon-style "ka's" and "nano's" from the actual idol VA's. Thirdly, if Puchimasu does prove to be popular, they've already spent the time auditioning and casting the right VA's for all of the idols! That would make it very easy to branch off into the flagship anime.
I also don't think it's fair to assume that a localized iDOLM@STER will be so marred. There's no reason to think that they'll move 765Pro to New York, make them all top pop stars, and re-name Hibiki "Echo". They probably want the English version to be popular with the large fanbase overseas already, and keep the same formula that attracted all those fans in the first place.