In Contractions^x, Proper Grammar^x

I'm a grammar junkie, and there is one thing that is bothering me with contractions. The other night I was making the bed and said: 'Why don't the covers stay on?' and I thought about it and figured that had to be improper grammar because the sentence really says: 'Why do not the covers stay on?'...

Or let's say, both sentences say that it was not necessary for him to go to the meeting, but with "didn't need," he either went or not; it was uncertain. However, with "needn't have gone," it is certain that he went to the meeting. Be aware though, that if you use proper grammar, you shouldn't...