About the multiple versions, the problem with "classical" package managers is that you have to do some manual renaming to ensure that both versions won't be installed in the same place in the file-system, which is tedious and doesn't scale well. Furthermore you may encounter some problems because both will be visible at the same time if the packager isn't careful enough. For example, if a software X has a dependency of libfoo2.1 and you happen to have libfoo3.1 also installed, the installation script for X may use libfoo3.1 instead of libfoo2.1, in which case you risk to encounter bugs because X hasn't been tested against libfoo3.1.