I like Shaq off the bench. Right now, between Nate, BBD, and Wafer, we have a lot of jump shooting off our bench. Marquis, however, is the only guy who gets a lot of inside baskets. I think putting Shaq with that unit would be a better fit, giving the bench a focal point to run the offense through.
I see the arguments both ways. Shaq's defense could be better masked playing with the starters, certainly, and the team could use his rebounding. However, his offense would be largely muted, as we generally run our offense through Rondo and Pierce, and I can't see that changing. Conversely, J.O. is a jump-shooting big, which hurts our offensive rebounding, but which should open things up for Rondo and Pierce driving to the hoop.
It's something I expect to stay fluid. J.O. will probably start the majority of games, but on nights when the team can't hit a jumper, I'd expect to see Shaq mixed in with the starters a lot, etc.