The difference between being "good" at something and excelling at something can often be found in the small details. Sure, you can be "good" at playing any PC game with mouse acceleration on. However, if you have it off, you now have a 1:1 ratio and can make use of one of the most powerful abilities humans have: muscle memory.
The difference between the two cases is enormous:
1. If I move the mouse X distance at Y speed, I get Z result.
2. If I move the moues X distance, I get Y result.
Human muscle memory takes incredible advantage of the second case. It is so much simpler for your brain and hand to coordinate in the second case. Sure, you can teach yourself the 1st case, but you are just making it more difficult for yourself. Removing barriers of difficulty in any task will help you perform that task better.
Having mouse acceleration on is, in my mind, akin to only using odd numbers for hotkeys or disabling your "a" key, forcing yourself to click. It just makes the same task less intuitive and more difficult.