I thought of this tweak when the iPhone first came out. It was annoying and slowed my maneuvering.
However I have to disagree with you. I bet some folks like having the folder stay open. After I downloaded the apps I needed and got my springboard broken in and folders arragend properly, I find that the folder staying open is intuitive and logical most times. However sometimes it is not...so therefore it really should be a tweak or option.....or perhaps that is what you are saying....that it should be an option as opposed to saying that they should have thought through the user interface for a better solution.
Either way, I personally think it is a happy medium, and i like it much better than the other solutions such as categories, pogo plank or karakae or whatever it was called.