There are people here who have come up with all kinds of reasons to dislike quill stems, but it's mostly hyperbole. If threadless stems are stiffer, it's at an order of magnitude impossible for mere mortals to detect. Maybe they're slightly more versatile, since you can flip them. Is this really such a big deal?

There are advantages and disadvantages to both. Neither is clearly superior. I suspect that the primary reason the bike industry has moved to threadless is because it makes things cheaper to manufacture (even though consumers have never seen a price drop that reflects this).

The way I see it, if threadless was truly superior, there wouldn't be so many bikes still being manufactured with quill stems. Think of any other 20+ year-old design improvement on bikes—if they were any good, they became 99% standard.