Well... I always heard that "racism" meant the idea that one race was superior - often by the intimation that another race was inferior. Similarly with sexism, ageism and heterosexism (the last being the most blatant example of "the named thing is believed superior"). It fits in with altruist (one who believes that altruism is desirable), and other such constructions.

So for me, ableism means that being "able" is better, and people who aren't are inferior.

But it's the nature of language to change. I have no problem with disableism, from that perspective.

But I think it's useful to think of the origin. Someone will say "I have nothing against black people, but they are in some way inferior to white people because they haven't done enough to improve their lot." Pointing out that racism is specifically the assumption that (in this case, white people) are superior because obviously everyone has the same opportunities, but only white people take advantage of them, can be useful.