There is no reason to have multiple versions installed.
Well, I don't want to open a debate on this but only which are the reasons beyond this wish
I've several aplications in production (not commercial thought), packaged with B4Jpackager, and running from users around the world.
It's easier to control exactly where and when I have to upgrade this app from one version of B4J to another, and which current version I'm downloading,running,etc.. so possible errors are minimised with these users.
When you're working with several versions of B4J all help is welcomed
On other way, when someone wants to download any application from Internet, usually the final user wants to know which is version of the application (from the Download section on the web - check VS, Arduino software, etc,.) This give the program/webpage a more professional look (or a designed webpage taking in account final user)
Even the downlabled filename could be changed for anything more readable that B4J.exe -ex: B4J731.exe)