I'm not sure if Wallace next to Josh Smith or Rashard Lewis works very well. I understand the desire for a "defensive center", but Wallace's defense has fallen off a lot, and I think that would be even moreso without any interior presence.
For whatever it's worth -- probably not much -- Detroit's defense is substantially worse this year when Wallace is on the court than when he's off, both in terms of points allowed and eFG% allowed. His "Opponent's PER" numbers are pretty bad, too, and his Dmult is pretty much league average. Giving Wallace the benefit of the doubt, he's an average defensive center, and maybe the worst offensive center in the NBA.
Roy if you're gonna just make stats up.... i'm out... 
What in holy Moiso is Dmult?!?!
Defensive multiplier. It's a measure of an opponents production, looking at how opponents do compared to their averages when facing a certain opponent.
Wallace's Dmult is something like .98, which means that he holds opponent's to 98% of their average production. KG's Dmult the last two years was .88 and .79, for perspective.
Like all defensive stats, I'm sure it's flawed. However, there isn't a single defensive stat out there that suggests that Wallace is still a good defensive center.