I think at a time where good bands are offering their music for free download on websites like myspace it can get quite hard for a new band should they decide not to jump on the bandwagon. This might also have to do with mp3players becoming afordable for just about everyone during the last years. In my opinion getting on peoples mp3players has become somewhat crucial for a bands success. So at a time where many people have GB of music on their harddrives and mp3players, listening up and down their playlist all day, you really don't want your songs to be excluded. If you expect a person to visit your gigs, buy your albums, whatever, you have to get his or her attention first. Not allowing people to listen to your music when and where they want might be quite risky since there are so many other good bands willing to do so, not to mention that most people know how to get/steal all the songs they want anyway.
I play in a band myself and recently we recorded two songs, paying about 330€ each.. which is quite a lot considering that we're all still going to school or college. Still I wanted to make them available for free download for afore mentioned reasons, to get known, to make people give our music a chance. If more people like our music, whether they have payed for it or not, more people might come to our gigs. Bigger clubs, festivals, sponsors might get interested. I think nowadays it's all about reaching a large audience that opens the doors to a bands success and offering songs or albums for free (or for a donation) might be a good way of achieving that goal..
P.S.: still in the end I was outvoted by the other band members.. no downloading, just listening .. oh well .. ;-)
P.P.S.: what I wrote might not completely apply to bands like radiohead ;-).. I guess they just like the idead of free music .. give (if you want) and (or) receive (anyway)