I use octave. As long as it gives me the same answers as Matlab, I'm happy with it. I have access to Matlab, but only two licenses. With octave, I can run as many copies as I want on our computing grid without worrying that any jobs are going to fail because they couldn't check out the license.
Why not Python, etc? For the same reason that most matlab users would probably give: the research scientist that gives me code uses matlab.