The main reason I would put forward is that it's completely free and open source. That means that all your work is transferable and usable by anybody, not just people who have the ability to use Matlab (including possibly your future self).
http://stackoverflow.com/questions/3534867/why-when-should-i...
But really I just feel like (certainly in my field) the open source community around R is huge and thriving and it's a better bet than Octave in terms of that.