This is not the regular season, when you may be able to give bench players minutes, even when they aren't playing great. I just watched every minute played last night again, and he definitely struggled. Other than a nice offensive rebound, and a nice charge he took on Odom, he didn't do much.
The Lakers had adjusted to him, and stopped leaving him wide open (although I don't know if it was an adjustment to Powe as much as them playing better defense in general), which made him much less of a factor on offense, and defensively, he just makes too many mistakes when you compare him to KG, Perk and PJ.
When he plays well again (probably next game), his minutes will be back up. But there is no reason to leave him in there when he is not being effective.