It's absolutely true that those things aren't exclusive, but as somebody who often uses two rare but capable user-agents (Lynx and the browser on an old Nokia S

It's absolutely true that those things aren't exclusive, but as somebody who often uses two rare but capable user-agents (Lynx and the browser on an old Nokia S40), I can testify that in practice, a significant number of websites aren't being created with both in mind, and I think a significant number of designers don't really understand what the lowest common denominator really means. It's very very easy to pay attention to only a handful of user agents, and from what I can see, right now that's trending towards 2-3 desktop browsers + "mobile" (which usually means "the iPhone and maybe one or two Android devices").