I don't like Shaq on the second unit because that would be asking him to do too much (which is why he was ineffective in PHX and CLE). Shaq is money on the starting lineup right now because he is not being asked to post-up and create like in previous seasons, he is only being asked as a body for Rondo/big3 to lob to. This works because Rondo can penetrate and burn any defender, and because he plays like a true PG, he is always looking for the easy lob to Shaq. No defender wants to leave Pierce, KG, or Ray open, so ultimately the defender that collapses in the middle is the one responsible for Shaq. This is why he is so money right now.
On the second unit, however, he would have a much harder time being efficient and getting easy points. This is for two reasons. 1) Delonte/nate are no where near Rondo in terms of passing and helping teammates setup for an easy shot, and 2) Opposing teams are not afraid of leaving the person they are guarding on the second unit (like Daniels, BBD, Semih) in order to help get the ball out of Shaq's hand. Basically what this means is that the lack of proper penetration & passing from Nate/Delonte would mean that Shaq would have to return to being a post-up player, AND he would have to create difficult shots on his while being double teamed.
I'm not saying Shaq can't match up against any second unit center, but I am saying that right now he fits in with the starting line-up much more, and that if he became part of the second unit he would have to work much harder and have a more difficult time to get points. It just doesn't make sense.