Bethlehem Shoals has an interesting theory on why this mix of stars works. The thinking is that KG and Allen make up for Paul Pierce's shortcomings.
Anyone's who has watched Pierce over the years knows that, as with Allen Iverson, you kind of have to take the good with the bad. Getting Pierce more involved in the flow of the game means more disruption, awkwardness, and general confusion. For Pierce to simply excel at buckets-getting, some mess is going to get made. That's why pigeonholing Garnett and Allen is the smartest plan for the Celtics; simply put, they need to counteract all the unpredictability created by Pierce.