Why, may you ask? Because often times I have a bug report (hey, I'm on Solaris remember?) for some backend library that I don't give a fuck about, but which I need for something else, e.g. the fox gui lib, so that I can get FXRuby so that I can ultimately use an IDE called FreeRIDE.
So, I can either annoy the author(s) with direct email, which might piss them off severely, or I can subscribe, ask my question, hope for a response and then unsubscribe. Pain.
Another reason I hate mailing lists is that the archiving/searching interfaces for mailing lists usually fall into the "terrible" or "suck ass" category. That, or they're so overrun with spam that they're effectively useless and probably dead because they weren't moderated.