Right now I think the bench is inconsistent, rather than bad.
There's a good deal of talent on those backup units with Leon Powe, Tony Allen and Eddie House ... plus Glen Davis as a solid role player and defender.
I think the team needs time, especially Tony, to (1) mature as players (2) get accustomed to their new roles.
Eddie House is the only true veteran player on that bench rotation, the rest are young players. Leon Powe is in his third year but really only has half a season of experience. Glen Davis is beginning his second season. Tony Allen is in his fifth year but his career has been stopped regularly by injury, and his roles have changed considerably over time, so overall he's still trying to establish himself as an NBA player. This is a young bench who are still finding their niche and consistency.
In terms of roles changing - Tony Allen has the biggest adjustment and that's fairly clear, now a relied upon member of the second unit and first wing off the bench, big step up from last season. House has the second biggest now that he's the only veteran around without Posey, more responsibility to step into. Then you have Leon who is getting far more touches in the low post and being used as a go-to offensive player rather than a finisher. Leon is also still learning on the defensive end. Lots of change. Glen Davis is playing a similar role to last season.
You also have the changes from Tony Allen to Posey. Having a slasher versus a three point shooter. Having a 6-4 guard versus a 6-8 combo forward. Lot of changes.
To conclude, I think the bench needs time and will as both a unit and as individuals (Tony/Powe) continue to improve throughout the season.