I think you are looking at it wrong. It is not just "use unstable", it is "use unstable if you want to apt-get install stuff".
You can always install the version you want, you just have to compile it, including it's dependencies if needed (using a custom path for libraries).
If you need to do that for multiple machines, build your own packages.
You have a choice how you want to get stuff on your system.
People creating software packages that are in the official repository make it easy for multiple projects to reuse stuff and it works out for most people most of the time.
You can always install the version you want, you just have to compile it, including it's dependencies if needed (using a custom path for libraries).
If you need to do that for multiple machines, build your own packages.
You have a choice how you want to get stuff on your system.
People creating software packages that are in the official repository make it easy for multiple projects to reuse stuff and it works out for most people most of the time.