Because a seven footer who doesn't rebound or play defense and floats around the three point line on offense is a good replacement for KG, the best defensive player of the last twelve years.
Sure, the former ALL-STAR Rasheed Wallace doesn't rebound, play defense, or any of that. He only helped Detroit reach the conference finals six years in a row. He is only one of the best three point shooting big men in the lg.
Bottom line is, he can post up, shoot, play D if he has to, and he can rebound. His averages prove that much. If KG goes back down, he will be a ten times better replacement then Big Baby. Celtics will be fine.