I had a longer reply, but I was rambling so I shortened it, believe it or not, heh!
I book small venues- bars that draw crowds of 50-250 folks depending on the night, the draw, and a million bits of chaos theory that controls crowds of people, hah. I would have to say that for those types of venues at least, it would be very difficult to come up with stats- I know just in the downtown area of the city I book in (Rockford, IL) there are 30 bars, half of those at least occasionally have live music acts, about 5 of those have live music more than once a week, and that the term "booking agent" has a wildly varying meaning in the small time venue scene, from the worst- some guy who gets calls from bands, puts their name on a calendar somewhere and thinks his job is done- to people who actually put some work into it... like me ;) - dealing with venues, booking acts and finding bands that will match alright together, designing and distributing fliers, making sure the bands questions are answered and concerns are addressed, setting up PA and running sound, getting the band some snacks, making sure the bar gets the bands some drink tickets, etc.
Sometimes a venue is a good-sized place with a stage and a solid in-house PA, sometimes it's a dive bar with a corner cleared of tables and the band providing their own PA, so that varies kinda widely, as well.
In my town, it depends on the part of town you're in as to what the genre breakdown is. I suppose what's loosely referred to as nu-metal does well on the East side, but'll bring a crowd of about 5 to Downtown. And the types of music being represented not only depends on what the local bands seem to be doing, but also on just how crusading and eclectic the local booking folks are- and I personally pride myself on bringing in bands that run the gamut from rock to punk to hip hop to "outlaw country" to folk to techno, and book bands not only from around the country but on occasion international acts- though I would have to say a majority of the bands that are available to me in my area of the world fall somewhere within that amorphous and barely defined genre of "indie rock"
As correctly but also kinda ridiculously pointed out by Eliot Van Buskirk in a WIRED article- "If indie music were a major label, it would be the biggest in the world"- what does this mean? Well, I think it means everyone has a bit of their own definition of exactly what constitutes indie, what makes up a music scene, how many shows and what kind of set up for live music have to be at a place to make it a "venue"... and one of the toughest questions, with so many bands with cross-genre appeal, just how the heck does anything break down into genre? heh!
So... I guess we need to define our terms, or just leave it wide open (which I'm pretty okay with, too!) and see what people have to say about everything from their cities local 10000 person venue to their favorite dirty dive-bar act :)
I'd love to see some more feedback on this, and maybe I'm totally off base and there's more solid info on small venues and small agents and small bands than I might think!