I read a note somewhere on PAUSE about how "CPAN is getting large, please delete old versions - they will be on backpan" ( Cant seem to find it anywhere now ).

I have also noticed that Google sometimes shows the older module above the later version - so its possible that a visitor from Google ends up on an older version of the module.

I generally remove all old modules ( other than the previous one ) - The exception of course is when I stop providing backward compatibility.

There seem to be other reasons for removing older versions which I don't really subscribe to.