They thought he would have more movement on his fastball and still an effective velocity difference from his offspeed stuff. This would result in fewer pitches/out and allow him to go deeper in games.

FWIW, this is what Lincecum has done. He throws very few 4-seamers, relying on the 2-seamer and movement.

I agree with you about people's perception of Posey. He has adjusted quickly at every level. I'm sure he'll start slow in the majors, but he'll be fine.