The difficulty with this is that a release, even an pre-alpha release, is a specific snapshot of the code. You can know (more or less) what issues apply to it, what features it does/doesn't have, and what is compatible with it. But a dev 'release' can change substantially over time and it's not easy to see what 'version' of it you have except by comparing the datestamp and scouring the commit logs. It's much easier to deal with something that is 'compatible with pre-alpha 3' than something that is 'compatible with the dev version, but only with copies dated between July 3rd and November 7th, except for the ones between August 12th and August 24th, which temporarily broke compatibility.'

Not as much of a problem for stock use, but on a heavily customized site, maintenance can be quite a headache with dev modules.