First, as a special education teacher, let me point out that the majority of children who are defined legally as "disabled" do not differ in any major way from their peers except in their ability to learn, especially in terms of reading, writing, and mathematics, and these students can be very successful in any physical activity - often more successful than they are in academic tasks. The incidence of students with physical and/or cognitive handicaps sufficient to make them stand out in such settings is very low; mental retardation is about 0.1%, and orthopedic and other physical disabilities run about 2.0% in children in the general population (statistics
here).
I have two adult students who have disabilities who fall into both categories above. As a special education teacher, I have, perhaps, a better background the many instructors in helping them to succeed in class. Both of these students (adults) have cognitive delays (in their cases, IQs in the mid-60s, compared to an average of 100); one has cerebral palsy and one has Down's Syndrome. Will they ever meet the same standards as other students? I don't think so... but the one with cerebral palsy has been in the class for 5 years and is halfway between 8th and 7th gup in a system that starts at 10th gup and goes up to 1st gup, then to black belt (ranks have been modified for both of them so they can see their progress). Now, non-disabled students generally get to black belt in 3-5 years, and he is still a yellow belt - and he's still there. That's perseverance, of a type I've seen in very few people. He's not a perfect student (his social skills are about equal with the average 6 year-old), and he will never perform at the same standard as others his rank - but he doesn't quit. The other student, the one with Down's Syndrome, has been in class about 1 1/2 years, and is the same rank; his coordination, while not great, is better than the other student's, and his social skills and ability to pay attention is better.
On the flip side, they slow the class down. They require more repetition than anyone else, and there are certain things that I only do on days that they are absent. They are frustrated, frustrating, and, in the case of the student with cerebral palsy, disruptive. They have, I have no doubt, caused other students to not join. On occasion, they drive me nuts. What my other students and I have learned from them is probably far more than these two men will ever learn from me, and even when they make me crazy, I appreciate them and the lessons they have taught me and all of my other students, and I wouldn't have it any other way.