Threaded is always better for adjusting fit. That is it's only non-aesthetic advantage.

The migration to threadless wasn't just about marketing, or cutting costs. Threadless dominated from the mountain biking very quickly because it was simply better and more robust. I don't think I've had my handlebars whacked out of alignment once since the time my old mountain bike got switched (in the course of getting my first suspension fork) in like '97 or something. I mean, when you're dealing with an expanding bolt vs. 2 bolt clamp you don't have to be an engineer to see which one is going to be more reliable.

Road bikes took longer to transition because there isn't an obviously felt performance improvement with the upgrade. I suspect the lighter weight in such a weight conscious industry was the kicker, especially as it made practical to make steerers out of lighter materials.