support side by side installations of different maven versions

If you like existing users to adopt new versions of Maven faster and if you like to gain new users of Maven from the paranoid type of build dudes...

then I suggest you allow different versions of Maven to be installed and used in parallel.

For example one might have the latest stable release installed, but likes to give the latest beta from another a branch a shot. I.e. you want to be able to do your work as usual on top of the stable release, while building something on top of a beta/branch version of Maven at the same time. This does not work (at least on Windows) - in fact, Maven won't even startup or emit version info anymore, if two or more versions of Maven are installed in parallel.

Another decent example is this:
Have the latest stable release around and ready for your work on a daily basis, while at the same time keeping around an ancient version of Maven for some complicated legacy projects in maintenance mode. This is useful, if project management does not allow to invest time into migrating build configuration and subsequent testing of its results.