Right now, when you hold the middle mouse button in game, the camera moves like it would on a text document if you pressed the middle mouse button. This is only really useful for scrolling long distances where you cannot instantly move your view from where you are to where you want to go. In other words, its good for a text document, but not a game where you could have just clicked on the minimap in half the time.
At the moment, I can't imagine more than a few players use the middle mouse for camera movement. In fact, I'd be surprised if most could even tell you what the middle mouse button does in LoL if asked impromptu. It's too hard to control. You either zoom past something close, spend way too much time carefully getting the exact distance right, or could have just clicked the minimap faster.
Camera controls are really important for strategy games and those that share a similar control scheme, like LoL. The best camera controls I've ever had to work with were in Company of Heroes, where specifically, holding the middle mouse caused you to scroll the camera opposite the direction of mouse movement the same distance you would have scrolled the cursor - like the grab function when you are zoomed in on an image document.
Obviously, you can just move the mouse to the edge of the screen, and some players may not see the utility of what I suggested over that, but when you play seriously, split seconds matter, and what I suggested is much faster (about 1/3 to 1/2 the time) than panning the camera by moving the mouse to the edge of the screen.
TLDR: Make the Middle Mouse button work like a "grab" tool rather than a scroll tool. It would be much more useful, and work better than normally panning the camera.