Beleive me.. Maintaining a working 'testing' distribution is much much much easier then maintaining a latest Fedora setup.
The way things work the packages are designed to work like this. In fact doing what I described works better then trying to be a 'pure' testing user. If you 'pin' packages so that you tell the OS to prefer 'testing' over 'unstable' but allow you to install 'unstable' packages then you avoid a lot of pitfalls that go along with package management.
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And back porting newer packages from testing to stable, by recompiling packages from source packages allows you to use a stable distribution, but selectively upgrade the specific peices of software you need that may be too outdated for what you need. For example: if your tying to run a PHP website you downloaded off the net, but the mysql PHP packages from Debian stable are not new enough then it's easy to selectively upgrade those specific packages you need.
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Basically: If you think this sort of thing is very hard to run and maintain then you have not done it.