I am a very happy user (in 98% of the time) of Python/Numpy/Scipy (anaconda distribution). I switched from Matlab and do not regret the decision. I have reached a level of expertise, which enables me to help others with their computational tasks and encourage people to use Python.
Unfortunately, the situation is not that easy. A senior researcher in a very closely related working group is using Mathematica. One of his students is using Mathematica, one is not sure yet but honestly it would be stupid to use a different software.
If I switch to Mathematica, everyone may benefit from it (code sharing and building up knowledge). I think you can understand that my motivation to do so is not the highest (not again another language; I use Python, Fortran and a bit C++). Additionally, the Python user community is very vibrant, and they frequently come up with interesting projects.
In order to avoid a nonsense discussion about what software to use, let me rephrase my questions as follows:
Have you ever been in a similar situation (either as student or supervisor)? And if you have, did you try to get everyone to use the same language? Did any situation occur where it was good that not everyone was using the same tool?
My field is biotechnology. We do calculate: ODE'S, PDE'S,fractals, system of equations (ODE's, DAE'S, algabraic). Most of the time we do some rapid prototyping (e.g calculating linear pH gradients in chromatography, some combinations of reactors etc., using/extending chromatographic models). I am a PhD student and will continue as a post doc at the same institute including occasional lab exchanges abroad.