No it's not used by hardly any games, it's used by zero games. The button is only there for legacy controls. I believe dev's don't have access to it.
I know my thread's a failure :(
Maybe it's true that developers can't use it for 3DS games, but I'd have to look more into it. Right now I don't think any of my 3DS games use it, but most of them aren't in need of an eighth button (or twelth, if the d-pad is used as four buttons). I haven't really thought of it before. Not that it matters though, because there's no drawback in having that button on the system (and it's, like you said, necessary for backwards compatibility with DS games).