In my opinion, matlab's excellent object inspection and debugging capabilities can be replaced with strict testing standards in your code-base.
On top of that, I get to use a whole slew of libraries that are non mathematically related -- frameworks for web services, accessing ftp servers, sending e-mails -- a lot of automated "utility" stuff.
And it is all "free". Fantastic.
but the main one for me, was that python does non-math things much better than matlab. Since python is a general purpose language you can go from analysis to production application much faster, whereas with matlab it usually involved getting a software developer to rewrite it in java.
We used to take our python analysis code, wrap it up in a web app, and then use that to server risk information to traders, and it was quite easy to do so.