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Because of the right-drag thing, Windows can't immediately display a right-click context menu on button press. It has to wait for the button release.
That's how it should be! You have to give a way to the user change it's mind before doing anything.
Also, middle mouse doesn't look like a good solution with users emulating middle click...
...and we should not forget too that middle click usually does some other default action in most of applications (looks firefox and page scrooling and tab closure with middle click...), or are used for fast document scrolling (also, firefox for example)
...also, (almost forgot!) the middle click in MANY mouses today is the scroll wheel pressed... so, how can I not release the wheel and ALSO use it to scroll down (example) the screen and release the documents I want in the folder in the bottom of the scrolled screen? Do you see an evolution of ideas? Combining simple behaviors naturally without giving up any of them? It's like typing, picking up the phone and still typing with the other hand (and probably getting information for pass on the phone...).