I replaced a threaded fork, headset, quill stem on my merlin extralight with a threadless headset, carbon fiber steerer tube. It saved over a pound of weight off the bike. Also the stem is much stiffer than the quill stem was.
Big advantage of a threadless set up is that it allows carbon fiber steerer tubes.
I have to agree that the quill stem is more attractive. The Cinelli stem on my Paramount is far more graceful, than the Newton on the Merlin.