Honestly, FB/Twitter isn't enough. It's just used for connection/promotion.
With a website you can post proper blogs, have your discog, links to stream / buy stuff, tour dates, lyrics, media (photos/videos), any necessary contact info, etc. I also like bands that have their own community attached (Jimmy Eat World is a good example - http://bbs.jimmyeatworld.com/forum.php
) As someone in web design though, of course I find having a website important. Even if it's a simple website with contact info/news/links to social networks. It looks better that way. It also gets you #1 in search engines above your FB/Twitter pages.
As previously mentioned, tumblr can be utilized as a website. At least then you don't have to deal with having a host/using cPanel etc. I mean, for a band's site, all that stuff isn't necessary anyway. Just have a domain so your URL isn't band.tumblr.com, customize the layout and you're set. Example: versaemergemusic.com / theswellers.com (re-directs), etc.