I don't think speed is the only reason for using it - from my perspective in this context its more like free (in both senses) drop-in replacement for MATLAB, but with less "weirdness" than MATLAB (and Octave) that is also fast.

I also think any "immaturity" is only really in the package ecosystem, especially after the release of 0.2. Even then, the package ecosystem is strong for the things that people really need (naturally), and of course you can call any Python package from Julia if you want as well.