I think that when first starting learning to do a form, the applications aren't so much kept a 'secret' by the sifu, as they are seeing if the student can form their own conclusions on how a form could be used.
Someone telling you how something can be done in certain applications would certainly place limits on that particular movement
I don't agree, when i was a kid i was told the best way of how to use a crayon, but i still picked my nose with it, poked the dog, and drew on the wall.
Gee, I've never wondered about the application (or an application) about some posture in a form for more than three seconds. I ask, end up on the floor after it is demonstrated, then we try it a bunch of times, then I know.