You practice perfect form consciously to give you a good base for dealing unconciously with imperfect situations.
It's like football practice. You spend hundreds of hours working on positioning, leverage, foot placement and whatever else so that you can play minutes in the game in situations that are only a facsimile of what you practiced. If you're thinking about where your feet go when you hit the field don't worry, you'll be back on the bench soon.
I've always thought it similar for self defense training. You get trained in whatever stuff you can, and when the time comes it either pops out (and you win) or it doesn't (and you lose).
"Me fail English? That's un-possible!" --Ralph Wiggum
"A woman drove me to drink and I didn't even have the decency to thank her"-- W.C Fields