You just haven't used a good one yet
I purchased a zCover a while ago and also hated it...it was stiff and made typing ridiculous. I liked the idea of a cover to protect the keyboard, because I have spilled a drink on an airplane on my macbook and it didnt fare well. I went out and purchased an iSkin, same issue...stiff, uncomfortable, so I just gave up. A friend turned me on to a company called KB Covers, I was hesitant because I thought they were all crap...but he said these were different. I went on their site and saw all the applications covers they had...what a great idea, I struggle remembering shortcuts all the time...so I bought the Final Cut cover and WHAT A DIFFERENCE. It is super thin and very soft when typing, I was surprised but very pleased. I have purchased a few of them.