I won't vote, because I'm no longer gigging -- but a few thoughts, if you folks don't mind...
For a wedding/mobile person, I would say yes without hesitation. Even if they don't find you that way, it can still be a major time/effort saver, using the tools available. Not everyone (client) will use those tools, but the ones that do, will save you time and money.
Bar/karaoke -- depends... Some folks like Joe C., Cam and Steve, probably get most of their work through referrals and regular weekly gigs, and may not need one. On the other hand, if they can have a place where their regulars can easily check where they're gonna be, it would come in real handy.
Sort of an analogy here:
Y'all know my favorite floating bar is less than 3/4 mile from my house, but I generally don't go down there on a regular basis. I check out their MySpace page, to find out what bands are playing on what nights. Very handy
Anyway, in this day and age, it seems sorta silly not to have one, since most people are on the net, and it's a very inexpensive way to keep your present/potential clients informed, give them tools, etc.
I'm sure many survive just fine without one, but why not go that extra mile, and show your clients you care
