In which case it's no different to Linux, OpenBSD or whatever.

As long as its your account files that matter, and you're running an app un-sandboxed, then it has access to them.

Nothing Windows or OS X particular about it.

As about "ruining your system", no, without root privileges it cannot, in either OS X or Windows. Of course it can if there's a privilege escalation bug, but there are tons of those for GNU/Gnome/KDE packages too.