The ability to score is the most important facet of the game, name one great player who couldn't score? Checkout the PER formula and see how much more scoring correlates to winning than rebounding, assists, blocks, steals.
Well, I agree any great player has to be able to score, but Wilt was the best scorer ever to play the game and didn't win much more rings than the defensive minded Russell, I suppose. And PER is just a compilation formula with theoretically derived weights. One of the criticisms frequently made to PER is that it overrates scoring. Others formulas, like Wins Produced for example, don't value scoring nearly as much. Personally I believe that PER overrates scoring (and puts an unjustified premium on inefficient scoring) and WPs severely underrates it (and puts a crazy premium on rebounding and low-usage scoring), but, essentially, that those formulas are kind of nonsensical to begin with and more misleading than helpful.
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Powe is not nearly as good as Millsap. He can't produce consistently as we've been seeing this all season. The reason why there's a certain feeling of disappointment towards Powe is because people raised the expectations too much since the end of last season. I understand the Millsap Doctrine, but that's only applicable to players who are consistently productive off the bench, not to those who keep struggling with half of their match ups. Powe has to improve a lot to be a starter in this league, I think. He needs to read the game better, to know when to pick his fights. He has to take better care of the opportunities that are already there for him; he's too focused on shot creation when he's not really that good doing that. He's scoring about unassisted 0.5 points more per game, but both his scoring and his efficiency have declined: from 1.71 points per shot last season post ASG to 1.57 this season; from 20.1 points per 36 to 15.3. He can't turnover the ball this much, because turnovers are so costly and he can't produce enough to make up for it.
Of course they look better playing along the starters, but that's true for almost every player in the NBA. The fact that Powe suffers so much from not having 2 starters playing alongside him is revealing that he's probably not ready to start.
(it's curious how many were saying in the pre-season that the improvement of Tony Allen and Powe would allow Doc to play a 2nd unit without starters - now the problem is that Powe and Allen can't be effective without being mixed with the starters... go figure).
To sum it up, Powe has to improve his feeling for the game. He's a natural scorer and rebounder with solid footwork on the blocks and the willingness to play physically, but that only makes him a much smaller and much less gifted version of Anthony Randolph. He needs to play smarter to give the next step.
To use a metaphor, Powe has a better but very private relationship with the ball, while Davis has a better understanding for the game environment - his teammates and the opponents. I also see Glen Davis as the best and more versatile defender and someone who doesn't compromise the team rebounding by much, so if Powe is struggling with his scoring (or both are), I th ;Dink I'd play Davis ahead of him. If Powe is comfortable with his game, what he brings to the table is just too valuable.
Anyway, it's difficult to predict how much better Powe or Davis can be. It always reminds me of Bobby Knight crystal glass.... I seeee... I seeee...oh, it's fading... just a second, just a second... coming back... ahahah, great stuff.
PS
http://www.youtube.com/watch?v=50LsvwmgJ7I&feature=related 
