I don't probably even remember all the functionality I've included with middle mousebutton on my A1200 (as I don't use it as my main machine anymore), but I'll try to remember few
The most important thing is that it is used to scroll areas in MUI. Scrolling web pages, big images etc in _any_ direction while MMB is pressed is very handy.
I also assigned shift key to MMB and it made scrolling in previous cases perfectly fast
In-line renaming in Magellan is good with MMB. Also button banks have one more option to click them.
I also used double MMB to bring windows to front (then they won't pop up witout intention like with double LMB could happen).
Then I used MMB+RMB to switch to next screen and MMB+LMB to switch to previous screen.
All these have worked fine together without overriding others. MCP is good to assign some of those features...
