yeah, it's an excellent question.
personally, i'm not a huge fan of the logic that you learn it that way to get the feel of the move. or programming your body. if the drill is learned over and over one way, but the conditions that occur in that drill don't occur in sparring (i.e., the guy doesn't leave his arm out), then it's a sizeable leap in logic to say that you can learn it one way and then practice it in sparring.
it seems like there really needs to be an intermediate step, as a couple of people have already suggested. an opportunity to learn the difference between the ideal drill and the sparring reality (and i use the word 'reality' in a broad sense).
take this example: opponent punches and i slip to the outside, cross to the body, and upward elbow to the tricep (charlie horsing the arm). looks good on paper. but in sparring, he's going to drop his elbow back into place as quickly as possible to guard himself. so perhaps a better response would be to slip, cross to the body, then parry that returning arm downward so i can downward elbow to the bicep or skip the limb destruction and come up to the face instead.
regardless of the merits of that particular example, though, the general idea is that without some intermediate experience on how things will go, you experience such a discrepancy in sparring that there's no frame of reference for the drills. so a person reverts back to kicking and punching. 'kickboxing' as some people like to say. which sort of makes sense, as kickboxing is often drilled in precisely the same way it's executed.
if the intricacies of another style are going to successfully translate over to sparring, i think there needs to be a more gradual and deliberate transition.
stuart b.
When you assume, you make an ass out of... pretty much just you, really.