I've read most of the posts, except the really long argumentative ones, and I have not seen a couple of points. First, I have both quill and threadlesss heads.
I see plenty of quill stems, whenever I go to Target or Walmart, and those bikes are built by bean counters. So if threadless is cheaper, why do cheaper bikes have quills, hmmm? Ok, what about the other end, if quills are really superior, why have I not seen quills on REALLY high end bikes? It seems to me there are too many quality advantages to threadless and the bike companies know this and won't return to quills.
By the way, as someone who purchases some older bikes with quills, I dread every time I buy a used bike that's only in fair condition and I try to loosen the quill. I hope the grease has done its job and kept the fork and stem from binding together.