I agree with you that spreading the field will fit better with McGahee's cut-back style and take some pressure off the O-line. At the same time, it puts more pressure on JP to prove he's a credible passing threat and keep the D honest. In other words, sort of opposite of what we heard in the offseason about having him just be efficient and not carrying the team.
The "experiement" of having JP run pure drop back sets did seem to fail miserably in the Detroit game. He was really locking in on his receivers when he took more than a 3-step drop and the DBs were reading his eyes pretty effectively. He's more comfortable on the move, but again, that's going to put more pressure on him to be on the same page as the WRs.
If they're going with a more pass-oriented base set, its either a sign they're confident JP can make it work--or an admission the power running game just doesn't have a chance with this O-line. Hope it's the former!