It's a sort of yes and no. They don't score big points on adjustability and it can be fairly long-winded to dial in a good position especially as there seems to be fewer and fewer variations of stem angles available these days. Don't get me started on the availability of polished stems either.
They made much more sense when you could change steerer tube on (expensive) suspension forks - so if you stuffed the frame up and replaced with a different head-tube length, or swapped headset with a different stack height, or swapped stem with a different stack height there was no potential big loss after you cut the steerer - this is where Nik's comment comes in
I think the introduction of Aheadset on road bikes was more from an economy point of view from the manufacturers and made things simple for bike shops rather than it being a technical necessity (like 1 1/8 forks too).
Aesthetically, it boils my blood to see logos upside down just so the stem will "work" flipped - for the love of god despite best efforts to make a lovely stem the end result is butt ugly compared to a nice Cinelli or Nitto quill stem.
Favorite quote on Retrobike:
Not worth the petrol to take it to the tip so it's down to the angle grinder to make it small enough to put in the wheely bin.