I don't think that difference between MATLAB and ROS is due to mobile robots or any other functionality, we should look for development/application point of view to get the difference.
I see MATLAB (Octave or Scilab) as a great tool for the prototyping phase where you are focus on develop control algorithms, in this step a powerful tool that MATLAB is very useful because allow you to focus on the problem itself.
After this phase there is the system development phase (design and implementation). In this phase you design and implement the code you have prototype in MATLAB and other functionalities that are necessary to make your system work.
This can be done in any platform (OS, program language, libraries) you want, your own platform too. However, doing this by yourself would require a lot of time.
ROS provides this functionality out-of-the-box, and a lot of extra components ready to use. In addition, there is a lot of people around the world using and developing it .