Why people don't test development distributions

One think I really like about Gentoo is how the "developement version" works. The user gets to list all of the packages where they would like to get development versions (which can be "everything"), and they get development versions of those and not of other packages. (Of course, this depends on the distribution supporting being in a mixed state, which is much easier with a source-based distribution than with a binary distribution, where you'd have to make some hard choices about which versions of libraries everything links against; there's still obviously some cases where some combinations fail to meet version requirements for dependencies).

This is really handy for being able to test particular packages that you're interested in, and being able to adjust these as needed. For example, I could positively identify that X.org server 1.5 (and not anything else) broke my arrow keys. Also, I could then say that I didn't want to test it any more, because I'd determined that it had problems and didn't feel compelled to suffer needlessly until there was a possible fix. (Actually, it turned out that I was accidentally remapping the wrong keys in a config file because keycodes had been renumbered, so the fix was to my configuration, but I could use my arrow keys in 1.4 while I figured this out).

I suspect that it would be really helpful to the process in general to be able to tell your package manager: don't give me a system where bug #X is known to be present. Then you could do your update, find that sound doesn't work, report the bug (or find the existing bug report), say that bug's a show-stopper for you, update again (causing that package to be downgraded to an earlier version), and find out whether the new version of your music player is broken or not. When the sound bug might be fixed, the package manager would try upgrading again so you could test, and you could either say that it's still broken (and again revert to the known-good version) or find that it now works.