Well, look at it this way. By adding Wallace, we're actually less vulnerable to injuries because if Perk or KG is injured we have a starter-caliber guy waiting off the bench.
Same deal for Grant - if Pierce is injured, it's a much smaller drop in talent to go from Pierce to Hill than to go from Pierce to Walker, Giddens, or Tony Allens. It's still a huge drop, but not nearly as bad.
So adding Wallace and Hill only makes us less susceptible to injuries. The better our bench gets, the less we have to worry about injuries.