The reason Perk was getting his was because he was hitting the glass, HARD.
When he gets offensive rebounds, the team expects him to go ahead and go back up with it, and that's exactly what he was doing. Further, KG and the gang got him involved early with a couple of easy looks under the basket.
The second half was a different story because the Pistons brought out their full court trapping and doubling defense. Perk is not as effective in these situations because he isn't skilled enough to help the team break the trap; also, the Cels were not getting good looks from the floor and had terrible spacing - this means that Perk wasn't getting rebound opportunities or easy touches under the basket. Posey became the better option at that time because he can handle the ball to a degree as well as open up the floor to help offset the traps and double teams.
I agree in that Perk needs to get more looks, especially when he is feelin it, but when the Pistons bring out their aggressive D, it's up to Rondo, Pierce, and Allen to quickly break that press and get into the offense. If they can successfully do that, than Doc can keep Perk in the game, if they can't, he needs to put in a smaller lineup - it's really as simple as that.