The aileron is actually pretty ineffective when extended out to the tip (the tip portion is ineffective) because the lift near the tip goes to zero. So, the portion of the aileron near the tip is unable to modify the flow in a significant way to roll the craft.
While you're certainly correct, my question is, HOW near the tip? And how does that interact with the lever-arm effects that cause an aileron near the tip of the wing to be more effective at producing roll rate than one near the root?
(I went back and found a picture of the plane where I cut the ailerons off at the tips. It was actually quite a bit more than 2" from the tips, more than I remembered doing. Which could easily result in a large drop in roll rate with out conflicting with the very tips being ineffective).