I'll try to explain the reasoning for the consensus wanting to add another center. Haywood & Diop are the only true centers on the roster, but they both are limited physically, which will lead to limited minutes (25 minutes at most nightly) and the possibility of sudden nagging injuries (a la hamstring pulls, hyperextended knees, soreness in foot, etc.) that will keep them out of the lineup occasionally throughout the rigors of a 82-game season. Tyrus is Tyrus, so inconsistency and untapped potential will always be a part of his game. Biyombo & Mullens are still both raw projects that have yet to learn but are hopefully progressing on how to rebound at the professional level, so we don't want to depend on them at the center position when they probably can't keep your typical starting small forward from going mano a mano with them on the boards. All that said, not to mention the foul trouble history of some the aforementioned, it's pretty obvious that they need more depth at the center spot. Undoubtedly, I also agree with you that we need a SF, but the thought of someone taking minutes away from the growth of MKG isn't something that you should shunned upon when you look at the frontcourt issues as a whole.
I'm assuming no one we get for roster spot 14-15 will be as decent a prospect as Mullens or Biyombo. So I say just play them at 4-5 when we don't have Haywood, Tyrus, or Diop(5 minutes a night?) in there. And now with Dunlap considering MKG a part time 4, that could mean even less minutes at the big man spots for the guys we already have.