No rudeness intended. Just a big mouth sometimes.
You can actually install software in its own protected environment. Many do this in their own home directory, and of course there is also the way of just installing the dependencies and compiling into /usr/local, which wouldn't be touched by the package manager either.
Compiling I've done. Stuff like gfce Ultra NES emulator gui, emovix, the Glide64 plugin for mupen64 to get a newer plugin version, little stuff like that.
A fellow poster, Issyer, used to have a web site guide (it's likely still up) to installing stuff like OpenOffice.org and even KDE into its own chroot home environment (something like that). So yes, we're running Debian but we do have users who sometimes like to live outside of the repository box and use different software.
I install the VirtualBox and VMWare binaries downloaded from their websites rather than just sticking to an older version of virtualbox-ose without the USB support and, of course, VMWare needs to be gotten from outside of the repos too. I do install them to /usr/local so as to not have them be interfered with by aptitude.
It's a GNU/Linux system. The bulk of things are available right in the repos, but we have the freedom to use the available tools to do other stuff if we want to. Debian has lots of tools to accomplish these things pretty easily.
So yes, you can have several versions of things at one time if you wish. It just takes reading a bit about the process and being careful. You can't expect aptitude to be able to keep track of that sort of thing, so no, you can't install both an experimental package and the one from your main distribution repo at the same time using package management, but you can build from the source or install into a chroot environment while still keeping another version in your normal environment.
It's not for me since I'm pretty much happy and willing to wait sometimes for things, but nothing's stopping those who may be interested from discussing and experimenting together. It's free software. Make what you will of it as long as the licenses (sharing changes, etc) are observed.