catalase addressed one of the points I was going to cover, so on to the rest:
It's a truism that in writing the first things you have to learn are the basic "shouldn'ts"--be they grammatical or characterization or world-building or whatever--and then you can play with the more advanced "shoulds" and figure out when those previous "shouldn'ts" can become "shoulds"*. People who make the "Well then I guess I just won't write any kind of [Minority X]!" response really aren't any more special than the countless ones who sulk over having to actually learn the rules of the English language, or pout over the idea that their fantastic cardboard constructions of characters might need a little rounding out. It's childish, and it's stupid to empower them by refusing to discuss the topic in question--those people are going to ignore you whether you talk about it or not, because they're not interested in learning. What they want is to be told that what they're doing right now is perfect, brilliant, and genius, and they're not going to listen to anyone telling them otherwise regardless of how it's phrased.
So, frankly, fuck 'em. They'll either come around on their own some day--many do, because it's a pretty common phase in the life of a writer; I had a horrible case of it myself at 19--or they won't, and in the latter case, why care about them? And if it is the former case, then there's all the better reason to discuss it, because when they do start realizing they could probably use work, they might remember the things they'd once dismissed and ignored (like, not to cite a personal example or anything, something their ninth grade teacher once told them about semi-colons). And even if they don't, there are plenty of other people that are, right here and right now, looking to improve, and will listen, and think about it, and try to apply what they've learned. Only they can't learn if no one wants to post things like this because they're afraid of the vocal percentage of people who will go "Well then I just won't write them at all, are you happy now?"
I think, thought, that it's easy to use something like this and say that anyone who isn't disabled writing about the disabled then uses them as a gimmick.
That's why I included those questions. They're judgment calls, yes, and a substantial portion of people will probably make the wrong call--and honestly, there is no hard-and-fast line, so some cases are borderline enough that any three disabled people might not agree if you asked them. However, plenty of people will wind up thinking about it and realizing they don't, actually, need to break that person's spine or cut off that person's arm or whatever to tell the story they want and need to tell.
What you have to ask yourself is: can the story I'm trying to tell be accomplished some other way? If the story you're trying to tell is the story of Lisa surviving Canary Wharf un-cyberized but paralyzed instead, then no, you really can't. If the story you're trying to tell is about the great hero St'reot'pe nearing the end of his Quest to Save the World when something bad happens and he is forced to take up the SuperShinySwordOfDarkness and imperil his immortal soul, then you don't need that "something bad" to be his horse throwing him and his spine breaking; there are so many other things you can use. But lots of people will do that anyway, because it's cheap and easy and handy to milk for drama.
If you can replace the character or event with something else and get the same effect, that's a gimmick.
* 'Round about here "should" stopped looking like a real word to me. More importantly, though: yes, this does mean that there are times and places where you can write a disabled character who would seem to violate every single one of those questions I asked, and still have it be a good use of disability. But as with any violation of 'the rules', the best way to make sure it will actually be a good and effective thing to do is to know why and when you shouldn't do it in the first place.