For advanced games, especially modded games, the middle mouse wheel is superimportant.
Multi-Theft-Auto does not allow joystick use; so one must emulate the keyboard. Because it negates the use of analog sticks, one must use the mouse to it's fullest in any immersive 3d game that does not allow my dual analog joystick. Usually in MTA my mouse wheel up is next weapon, and mouse wheel down is previous weapon, while pressing it is SPRINT.
The other hand is on the arrow keys. This allows the mouse/keyboard combo to perfectly simulate a dual analog controller. Without this, MTA wouldn't be much fun as you could drive cars ok, but trying to do anything outside of them is impossible(except for running to the next car and getting in).
Think about it...one hand on the arrow keys, the other on the mouse. I need three actions to be out of a car in San Andreas: Shoot, Aim, and RUN! Three buttons are needed!
Not to mention that as GE lacks ANY joystick support whatsoever, a middle mouse wheel is the closest thing we get to a real 'analog' input!
Why does this matter? Well consider your own experience with san andreas. How great was it? Mine is 10x better.
This is my Blista Compact.

- blistac.png (23.98 KiB) Viewed 3604 times
It is skin modded to be a Mitsubishi Lancer Evo, and it's max speed (so far) is 1,182 mph! I can't get it faster because.....there isn't a long enough straightaway in San Andreas to do so!!! This car handles like a dream, drifts so perfectly, yet powerful enough to jump from one side of the map to the other!
Point is, I take my MTA seriously! And MTA must have middle mouse wheel! One example!