Well couple of reasons. The time involved driving 90 minutes each way to the Apple Store. The fact that when I call they won't tell me if it is even in stock. Then, I don't want to get a screen that is worse than this one having dead pixels, dust and whatever. What's to say these new ones they are putting in aren't the same or worse. No one really knows yet that I have seen. So I'm not saying I won't do it but think I'll wait and see what people report.
So basically you don't let them fix it just because you Are not sure they can solve the problem .... Wtf ...
Your iMac, your call, but a very strange one in my opinion