I'm standing put on this one.
Every time I hear of splitting PP's 1st two quarters, 6 mins here and there, the game then de-evolves into him playing 38 mins. It never works out because in the end, Rivers doesn't treat a regular olde guy as a starter. This is a type of KC Jones mentality which then never goes away.
IMO, Ginobilli's a great example of this utilization by Popovich. I was really annoyed with the newcasters painting Tony Parker as the Spur's faltering *go-to* man when that was seldom the case during various playoff runs. In most situations, where the Spurs came up short, Ginobilli was nursing an ankle problem. When he's healthy, they're always a playoff threat. Manu delivers the goods during crunch time.
And likewise, for the Celts, the one man, who's never failed (with contained minutes), was Pierce. Thus, you want your X-factor in, during the final quarters when he's 100% rested and charged for the game. Realize, Ray Ray can't take it to the hoop, and get to the line regularly. PP has an instinct for this. If Ray comes off the bench, it's much like having a souped up Eddie House.