BBD is playing out of necessity, not because he is being showcased. The fact is, he has much more value to the C's, than any other team out there. The reason he is playing over Powe, is because Powe's rotations have been horrendous, and lately teams have been exposing it more and more.
Powe is always a step or two late on his rotations, and teams have learned how to beat the C's defense if they are not rotating on time. Davis is by far the best team defender (other than perhaps Scal) off the C's bench, so if they took him out of there, without replacing him with a sound defensive player, there would be even more of a revolving door to the basket when Perk and Garnett are not in there.
I think the best example (or most obvious of late) of Powes problems come from the drive by Lebron last night, where he beat his man with a crossover, and then Powe nearly fell over backwards, as Lebron crossed over again to fly past him. This was made possible, because Powe was about two steps late in blitzing the perimeter. If he had rotated like he was supposed to (and like Perk, Garnett and Davis do), he would have cut off Lebron's progress, by creating a wall with Lebron's man (who was actually showing Lebron to the middle where he expected help from Powe), and forced Lebron to give the ball up. Instead, he got embarrassed.
Now, I know pointing to a move by Lebron is not really fair, but this is not a one time thing. Powe is constantly making mistakes like that, that one was just a nice obvious one to point to. There were at least 3 layups in the Houston game, that were directly attributed to Powe not rotating on the backside, when the frontside defender showed on the perimeter, forcing the pass. Its a been a problem for a long time now, and unless Powe picks it up, he is going to be battling Davis (who is significantly less talented...but knows the defense) for minutes for the rest of the season.