I very rarely shoot from the hip, and when it happens is when surprised by an opfor and I don't have the time to shoulder the weapon, or similar situation. Needless to say, in these situation my rate of survival is pretty low, since hitting the enemy is actually much harder...
That said, I think that's the kind of thing everybody would know from the start, then again maybe not. Try to draw whatever without looking, it'll look like crap. Same thing when shooting.
About following the BB stream and adjusting, that's a valid reason that would make you ask that question, as indeed a lot of people seems to do so. It sure is easy to shoot at will when you've got 90-120BB+ in a mag... But if you pay attention, most more experienced players won't do that and take the time to aim, and shoot less BB in the end.
Try a few game in realcap and see how things go; You'll WANT to use your sight or you'll find out you're out of BB pretty fast...
:P