I find the button quite random. Sometimes it does its own thing.
I really feel Apple should revamp it and make its use consistent across OS X.
Only in iTunes. Seems that the re-sizing in other windows is just too inconsistent for my taste.
You would have thought Apple would have given us the facility to stretch a window from wherever you dragged it?
No, only in iTunes to get the mini player. Otherwise the window will end up like this: