I look at it this way. Powe is the better player and it really isn't close. That said, each can be equally valuable for this team. Powe brings scoring off the bench. Against, other teams bench players, Powe can score very well. BBD on the other hand really should be commended for working on his shot. It has gone from a prayer to reasonably reliable. He is by no means a good one on one scorer but within the team concept, he will be needed. The other thing is that his sheer girth is valuable for guarding bigger players. That is really the only whole in Leon's game. He can be overpowered by bigger PF's in this league.
In the end, both can certainly contribute to winning #18. That's all that matters right now.
I don't think most NBA evaluators agree with you. Many are split, at the least, to say Powe is the far better player and it is not even close is absurd. I think you are overrating Powe's role vs. his talent. Put him on OKC, and we would see the truth. I go back and forth on which guy fits the C's better, sometimes it is Powe, sometimes it is Davis, thankfully there is still room for both of them as they both bring positives to the table.
Not being able to cover wider bodies is far from the only hole in Leon's game. Leon is not capable of running the pick and pop, by far the most available shot for bigs in Doc's offense. The modified Princeton he runs, most often ends up in a dribble handoff from big to guard, or pass and pick, both resulting in giving the guard room to drive by the big popping, not rolling. Disagree with this style if you want, but that is what they run, and Davis is far more adept at this. Not just hitting the shot resulting from the pop, but reversing the ball adroitly and being able to put the ball on the floor once or twice to meet the guard for the handoff.
Coach K likes to refer to this as a "ball-friendly Big" meaning a big who is comfortable away from the basket handling the ball, reversing it, making good decisions, withstanding perimeter pressure without fumbling the ball away. Ball-friendly Bigs can be a huge weapon, see Garnett, and this is one thing Powe is most assuredly not. He is a beast in the paint and on the offensive boards, but outside of 10 ft. he is useless. They can never use him in the middle of the floor against pressure, because even with the slightest pressure, he will travel or fold up.
Davis on the other hand, has a far better floor game, is more versatile as an overall player. He will never have Leon's power game, but he is the more accomplished all around basketball player, even if not the better post player. Davis also has a great feel and finds the open man very instinctively. Leon does not demonstrate many of these instincts at all, in fact, he looks very uncomfortable when having to make plays for his teammates. He is very one-dimensional, which fortunately, as a bench player on a championship-caliber team, fits nicely with what is needed.
In my opinion, Davis is the better overall player when assessing the multiple skills involved in being a complete basketball player. Leon is better than Baby in a few areas, but Davis is reasonably good at far more things.