In my opinion a README should be a requirement for every project, be it open source, personal, or at work.
It's the essential piece of information of a project.
If a project doesn't have an exhaustive README, new people on the project may find themselves asking colleagues what the installation steps are, or even worse: search through the internal google sites wiki, or, god forbid, the journal!
Keeping a README up to date is such a simple procedure/practice, that it's a shame to hear the sentence from time to time "ah the readme is not updated, you have to do ..."
Come on, we have the diligence to keep a README informational and simple to avoid others unnecessary headaches :)
(PS: I'm the first to say that it's easy to criticise and to rant about things, but let's try at least to write a damn readme)