I hope to get a Mercian Super Vigorelli in the near future, and I intend to get it threadless.
Mercian has already told me they can and will do it any way I want.

I like threadless because it lets me experiment with stems.
I change stems all the time, looking for that last little measure of perfection in fit.
I have tuned my Bianchi Pista as much as possible, given the dimensions of the frame.
In order to go the next step, the frame has to change dimensions a little.

Regardless of fashion or aesthetics, I trust lugs more than tig welding or fillet brazing.
Yes, purely emotional, but nonetheless real to me.

A Mercian Super Vigorelli with a threadless headset: perfection.