this thought is drilled into our heads:
TECHNIQUES ARE ONLY IDEAS.
we learn a lot of different techniques so we have a lot of different ideas engrained (sp?) in our muscle memory, hopefully to the point where we don't consciously choose which technique to use for that punch flying at our faces, because by the time the brain registers the need to react, the muscles have already chosen a course of action and reacted for you...then the brain catches up and goes "oh. I guess that works."
we don't deliberately dissect techniques to change them. we dissect them when we're learning, and then we just pick a partner and the instructor calls out an attack, and when you hear "left hand punch" you better react with SOMETHING or you're gonna get hit. These drills teach you to react, and generally you don't end up using a perfect "technique" you use parts of them, but normally you just react.
if you're in the street, you're not going to end up with a perfect studio attack, so you can't use a perfect studio defense. you need to adapt, and drills like the one above teach you to move first and think later, because if you take the time to think, you've just taken a moment to dig your own grave. If you're attacked, nobody's going to care if you did "Shield and Sword" perfectly. They're going to care that you walked away and the other guy didn't.
just my $0.02