I've wondered about this, too. Certainly I've known people who've used MatLab effectively, and I know there's been discussion on this board about using R (the descendant of S), but SAS is notably absent.
I worked someplace that used it a while back. Once I got familiar with it, I have been able to use it for pretty much everything I have needed to do. The only thing I can imagine doing in the near future that it can't handle is automated trading.
My understanding is most of the retail trader oriented stuff is a lot easier for basic tasks but is not nearly as good for regression models, stats, and big portfolios. Depends on your needs and goals.
If I were starting from scratch now I might consider R instead. I also know some people who swear by Stata. But I would have to have a pretty good reason to switch at this point.