I find this concept of "ableism" or "disablism" odd, but it's par for the course in the age of identity politics. Part of my point of view comes from a few experiences I had as a kid. For one, on my Dad's side, my grandma, my Uncle Ed, and my Aunt Bobbie all work with the mentally retarded (and my grandpa worked with the retarded as well in retirement, mainly the trainable retarded.) I use the term "retarded" because "mentally disabled" doesn't really work when there are mental disabilities other than just low IQ, and the term is more precise. It's like many of the PC terms for various things - the new terms mean to obscure the real issue, and so can confuse. And my relatives use the terms "trainable retarded" and "educable retarded", and I've had enough experience with those categories of people that I understand it.
As well, in the summer between 5th and 6th grade, I volunteered at a Kidney Foundation camp, because my Aunt Pat (after whom I was named) got the idea for IBM to sponsor a program for the Georgia camp where the company would donate PC Jr. computers for ue during dialysis and during camp, and IBM employees would volunteer to show the kids how to use the computers. I came along, because Aunt Pat didn't know how to use a PC, and I was more helpful in teaching her than my Dad was (Dad was too technical and would tell her stuff she didn't need to know.) Many of the kids at the kidney camp had disabilities beyond their kidney diseases, mostly as a side effect of years of poisons not being filtered out of their blood and urine. Some were blind or had really bad eyesight, most were very short, some were deaf, some had mobility disabilities (in that they couldn't walk, or they walked very oddly).
Throughout all of this, there are a few things I've learned. The most important is that all human life had value, no matter how "useful" said person is, and that you should treat all people with respect. That said, treating people with respect does not mean treating everybody exactly the same. The problem with coming up with the concept of "disablism" is that you're veering into the territory that feminism trod in which people no longer feel obliged to give a bus or subway seat to a pregnant woman or an old person. Dammit, some people have physical problems and you should have consideration!
I'm not going to treat a mentally retarded person the same as any other person, because I don't even treat "neurotypical" people all the same. I'm not about to go on a lecture on metaphysics with someone with an IQ of 50. It's not kind. I don't even do that with people of high IQ if they're not interested in the subject (except for family members, because they've got to take it. And they can get back at me.) Likewise, I'm more likely to go out of my way to hold the door for someone in a wheelchair than someone who is in full control of their appendages.
So yes, ask for respect and consideration, but don't fight for being treated just like people who have the faculties you lack, because that can bite you in the butt. And it's not necessarily a good idea to argue that one is simply "differently abled", because you're still relying on arguing how useful a person is, and there are times in all of our lives where we're completely useless in various dimensions. I'm a staunch supporter of Not Dead Yet, and I think it's more important to emphasize the importance of simple humanity, rather than bitch about people treating you differently because you're different. Yes, most disabled people can do a lot more than is assumed (for example, the "educable retarded" mentioned before... in times past, people would probably not try teaching them to read, but my grandma did it for decades. Their reading comprehension was extremely limited, but they could still had a certain level of functioning that once upon a time would be considered out of read), and those boundaries should be extended. But the likelihood is that there will still be boundaries, just as "normal" people have boundaries for what they can achieve.
And the joke/riddle in my cut text is one of a series of limbless guy/woman jokes that I've heard in the "disabled community" (the answer to that one is Bob). Some of them were extremely tasteless (and evidently the staff of the Kidney Camp, many of whom also had disabilities, didn't think it inappropriate for a 12-yr-old kid to hear....but then, the staff were mainly college kids and likely had some judgment issues as well), but it's in the line of musicians making up jokes about violists or academics making up jokes about their own field. Too often the first thing to go when identity politics crop up is humor.