Yeah great, because KG can easily defend Dwight Howard, Al Horford, Shaq, Lee, Lopez, Kaman, Bynum, Duncan, and guys like Okafor, Okur... You get my drift.
KG can cover all of them other than Shaq (who is disintigrating with each passing game). And in fact, he is a better defender against guys like Lopez, Lee, Horford, and Bogut (who absolutely destroys Perk).
Perk is a better defender against Howard. In fact, he may be the best in the league at stopping Howard. However, KG, Wallace, and Davis have all had significant success against him in the past, so I don't think you can rationalize keeping Perk just to cover Howard.
Oh yeah, and then there is the fact that Perk is a black hole on offense, and at times causes more negative offensively than positive defensively.