BSD differs from Linux in some important ways. If one of those differences affect your works, than Mac OS X is useless, even if it is Unix (and Linux, some would say, isn't)
I get that, and I wasn't trying to say that GNU/Linux is Unix, either. In fact, what with systemd on most major distros these days, GNU/Linux is less Unix-y than ever (I'm not saying that's good or bad; I have my reservations about systemd but they are irrelevant to this discussion and I'm not being biased either way about it here).
That said, for just about any project I've ever worked on within a GNU/Linux ecosystem, I could have easily adapted it to Mac if that was my chosen platform. HTML is HTML, JavaScript is JavaScript, so for today's web-centric projects the OS you're running doesn't matter as much. Even languages like Python don't really care what's under the hood.
Of course, if you're doing Linux kernel development, or targeting a GNU/Linux deployment, then a Mac isn't the best idea. :)
"Useless" a strong word for a simple difference between two Unices that affects your work. You could just as well say Linux is useless because sysvinit and upstart aren't the same as launchd.
