Big men on the perimeter

I realize that there are times when a big man does belong on the perimeter. Our own Mareik Isom is a case in point. He is a perimeter player who just happens to be big. Also, there is some value to pulling the other team's shot blocker out of the lane to open it up for other players. However, in most cases, there usually is someone else who is at least as good at perimeter skills who does not do nearly as well what the big guy can do underneath. So, under ordinary situations, why waste your size outside?