I assume that there would be a way that administrators could disable per-user package installation if wanted. I don't see how the *existence* of the feature is a problem in and of itself. I'd certainly like to be able to install different versions of the boost packages into a local directory so that I don't have to touch Boost.Jam umpteen times to test against everything from 1.44 to the latest release. Or try out the newest shiny (especially with things like newer mesa where if it screws up, I still have a fallback to rely on). without having to upgrade an entire distro (or install an ancient one for older versions).
I would like to have 10 compiler versions installed so that I can test everything with one machine instead of needing 10 VMs of various versions of distros to get those versions and installing things by RPM is a whole lot more reliable than me trying to replicate what is installed in those systems manually.