I have encountered several CL libraries whose dev teams are either indifferent about doing official releases or have an explicit statement on their project website(s) notifying the world that they don't plan to spend time on such activities. One project in particular observes that formal releases are a lot of work and they can be troublesome, and hey installation is as easy as typing `darcs get blah...'
What these developers fail to recognize (or maybe don't care about) is that this attitude puts the next developer down the foodchain in a difficult position. Here's my perspective -- every dependency of my project is automatically a dependency for my users. When the current source control version doesn't work or whose API/behavior is changing dramatically from day to day, this becomes my problem. I am in a much better position to debug such an issue or look for a substitute, so I don't think it's fair to ask my users to do it. Obviously, I prefer to have some control over the situation, such as being able to say "the version of foobar I have tested for use with my project is 0.x.y patchlevel z". But when a project's release procedure consists of telling people to check out the latest from source control, I have little or no control over which version my users actually get.
So when I see a library that looks useful but doesn't do official releases, I have to decide amongst three choices: a) move on in the hopes of finding an equal or better alternative, b) bundle a snapshot of the library inside my own project, or c) nag the developers to make snapshots available that others can depend on being available (for some useful amount of time). None of those are good choices, frankly.
One last comment...I'm not complaining about brand new projects whose websites are barely a week old. In cases like that, one generally either sees a statement like 'this stuff is brand new and we haven't done any release yet' or it's just easy to tell. Every project has to start somewhere. My beef is with projects that have been around and whose developers really ought to know better -- this is basic configuration management.