This applies to people who can and do continue to work. You just need an SSI status that isn't likely to be resolved within X period of time. I believe depression and anxiety disorders are the most common cause of lifetime disability. Here's a comment from a Reddit thread yesterday, which mirrors my experience:
I have friends from high school who are on SSI for depression and continue to work for extra income. They're mostly lazy and none of them live what we'd think of as awesome lives. But I wouldn't in a million years think of them as anything close to permanently disabled. If someone put foot to ass they'd be capable of supporting themselves and probably better off and happier in the long run for it (which is a whole different discussion entirely).
https://www.reddit.com/r/news/comments/4eij5y/obama_to_forgi...
I have friends from high school who are on SSI for depression and continue to work for extra income. They're mostly lazy and none of them live what we'd think of as awesome lives. But I wouldn't in a million years think of them as anything close to permanently disabled. If someone put foot to ass they'd be capable of supporting themselves and probably better off and happier in the long run for it (which is a whole different discussion entirely).