I think part of why KG and Duncan want to be known as PFs instead of Cs, is because of the comparisons.
If you are a Center, then where are you on this list: Russel, Wilt, Kareem, Moses, Hakeem, Shaq, Walton, Parish, Cowens, etc? The first three guys are basically never going to be passed, and probably never matched.
As for PF - Kevin McHale is considered one of the greats, and he came off the bench for the first half of his career.
Duncan has been playing Center for most of his career at this point, so its kind of silly. For KG he's played PF his whole career, with only tactical exceptions, so I can see why he'd rather be PF if he's going to grind out one more season.
I think in the end he'll do what's best for the team if he comes back.