I really like Shaq used with the second unit because for first unit has no lack of offense.
Shaq gives the second unit offensive stability and the offense can be run through him. The screens him and BBD/JO will set for the shooters will be devastating, while he can post and hit the open man at will.
Plus, he slows down the game with the second unit in, giving the starters more time to rest...which is the typical role of second units.
The first unit has no lack for offense NOW. But in the playoffs, the first unit will need Shaqs easy scoring. Why?
Ray Allen will be in some sort of a slump(happens every year in the playoffs) or will be guarded tightly.
Pierce will have a tough cover(Lebron, Artest)
Garnett wont be attacking the paint like he should be.
Rondo cant shoot, and if they clog the paint he cant score.
The only option the first unit has against ANY lineup is Shaq, he can score against anyone, or will get fouled.
THAT is why he is important, to get us points in the early going and set the tempo so we dont start out in a hole.
Trust me, when the playoffs come around you shall see.