I've only used threaded stem my entire cycling life. The only headset crown race that developed indentations was the Campagnolo's of the 80's. For such a nice looking headset the crown races were not up to the standard by my usage running 25mm tires at high pressure. I have a new VO Rando frame to build that's threadless though. As for the funtionality, a headset is a headset. Adjustability is obvious as to the differences. As for availability, while complete quill stems are less available, threadless don't have anything to boast about either. Sure there are many brands available, but it's many brands of look-a-likes and spec-a-likes. I've resorted to buying UNO brand as they are one of the few that offer lengths up to 130mm and the potential for lower angles of -17d and below. Plus they offer them in silver. Limited stem choices, regardless of the headest implementation is akin to the limited choices in cassettes, where a 12t low is rare and a 13t low is even more rare. Not to meantion lack of cog choices. I see a certain irony in cassettes as when they were introduced a selling point was versatility in cog selection and inividual replacement. That flipped and now it's "you can have it anyway you like as long you like what I give you"
. That's not "progress", it's regress. But I digress, enough of that nonsense !
For quill stems and 31.8mm bars, I use threadless quill adapters and a threadless stem. They come in a variety of styles and quill heights. Sure it weighs more, but so what ? I've ridden enough bicycles of many weights to know that the weight of a bike has nothing to do with enjoying the ride of the given moment. Myself I've weighed more and less enough to know that such has no bearing on my Being.
Threaded/threadless are just two ways to do the same thing.