I think that ultimately, band-as-beer-salesmen is a failing model. It's just not that likely that a bar will consistently sell enough more beer with a band there to justify paying the band decently, compared to what they would sell with no band and no expense for the band. Bands are fragile things that break up easily, it takes time to build a following, and by the time a lot of them have been around long enough to have gained the experience to be any good and have a following, they're mostly middle-aged and so are their friends and probably not hard-partying kids any more. That means a pretty high risk for the venue that the old reliable bands will have broken up and you're constantly trying out new bands that don't bring the extra sales in you'd hoped for.
This has not been my experience. The core Redhead fan is female, divorced, 35-65. Followed closely by men in similar circumstances. They have the money and the time (kids are either gone or old enough to not need a babysitter) to go out on a Friday or Saturday night (sometimes both!) and have a few drinks and work on their flirting skills. Lots of couples, too, of course. And sometimes (to my great astonishment) we get college kids. Remember, they grew up in homes where mom & dad played Zep and Heart and Doobie Bros, etc. (Kinda scary to think that "Whole Lotta Love" was released 50 years ago, eh?) Is the band really good? Not for me to say. But we seem to have convinced a lot of people that we're worth seeing.
It helps that our BL is extremely outgoing--if she spots a group she's never seen before, she's going to take the wireless mic to their table and sing to them. At the break, she's going to go talk to them. And little by little, she's built us a solid fan base. The main thing, from a commercial perspective, is that club owners know we are a reliable draw, that we do a lot of promotion on our own (there's one guy who now refuses to book bands that don't do their own promotion), and that we do all the little stuff, like load in as quickly and unobtrusively as possible, get the crowd to cheer for the bar staff, keep the volume appropriate, etc., etc. As I often say, it ain't rocket science.
Edit: Nearly forgot. We get a lot of Girls Together Outrageously. They like Jane. They are almost always DINKS (Dual Income No Kids) and they are often the first ones on the dance floor. And when the Melissa Ethridge tunes come up, they sing along. Loudly. Love 'em.