So I guess it's more a stylistic matter that *all* late '80 and early '90 MTB stems pointed upwards, since a negative stem doesn't seem to hamper control.

Maybe I was looking too much into this, trying to find some esoteric point which is not. Probably the logic behind positive stems in vintage MTBs was only to differentiate them from roadies... road -> negative because wind is bad, off-road-> positive because... well comfort & relax.