Bar band gigs are a funny thing, and bear reflecting on if we want to take the complexities of the relationship between performers and audiences seriously.
Unlike a concert, where the patrons are paying (assuming it’s a concert one pays to hear) to experience the performance in the company of other fans, a bar gig is not actually an end in itself (so to speak). If you’re playing in a bar, your real job is to sell booze. The music is just a means to that end. Bar owners pay to have a band on site (and charge a cover to help pay for it) because the band is expected to be a draw — the music should bring in customers (hopefully new customers), it should give them a reason to stick around, and it should promote an atmosphere in which those customers feel the need to buy more booze. Unlike a jukebox (constantly on-site to lubricate customer interactions), a live band at a bar is a more active sales tool. Nobody comes to a new bar to see the jukebox. New patrons do, however, sometimes follow a good band with a decent fan following.
Patrons at bar shows of this kind  may or may not actually care very much about the music. Gigs of this sort are the bread and butter of cover acts, using the familiar tunes to provide a soundtrack for the patrons’ own good time, and as long as they play competently and don’t mess with the expected musical formula, the audience is usually pretty forgiving (mostly because they aren’t paying very critical attention). The patrons may talk over and through the songs in such cases; sometimes, if the band is good and the crowd is right, things get crazy and fun on the dance floor. Typically, a bar gig is not a good kind of show for a band showcasing original music that patrons are expected to listen to more closely (although in some cases, for bands with large and active fan followings, it can work pretty well — some of this is regional, which means that not all markets can or want to sustain original acts rather than cover acts).
Somehow, in this context, the band has to find a way to engage the patrons, both artistically (assuming they care about that) and commercially (in order to earn their keep). That means taking requests (whether you want to or not). It means sometimes doing pseudo-karaoke and letting a patron or two on stage. It means choosing a set list based not on which songs are most musically interesting or challenging or beautiful or meaningful or good for showing off one’s talents, but rather based on what experience suggests will get dancers on the floor and (especially) orders at the bar.
Where, then, is the aesthetic value to be had in a bar gig, from the POV of the audience? Sometimes, it may actually not lie in the music itself at all, but rather in the skill of the band at curating, presenting, and engaging the crowd with music; in this respect, aesthetically speaking, a live band is quite a lot like a live DJ (as opposed to a jukebox or streaming radio service). Where the band differs from the DJ is in the nature of the performance; a DJ is more curator and mixer than a band is, and a band must perform musically in addition to the task of curating, mixing, presenting, etc.  A bar band performance that might, in a concert setting, be quite strange and perhaps uninspired can be absolutely brilliant in the right bar setting, curating the right set list for the right crowd.
Think of it as the great lesson of Huey Lewis and the News, a bar band that also happened to find a career outside of the bars with original material. You don’t look to Huey et.al. for the next prog rock hit or something complex, hard-hitting, morally interesting, etc. You look to them for a good time — and their career was based on doing just that and doing it very well. This doesn’t mean that their music is not aesthetically valuable or interesting (I will make no claim one way or the other on that point here). It just means that for some gigs, whether the music is really brilliant or not doesn’t matter as much as whether or not the audience is engaged. 
1. There are concert venues that have bars attached to them, or that serve alcohol, but playing those venues is more like playing a concert than a bar gig — in those instances, the booze is just concession business, not the commercial point of the activity.
2. I do not mean to suggest that a DJ is just the same thing as a live band, or better than a live band, or anything like that. What I’m pointing out here is that the value of the kind of performance in question, at least in some venues, has more to do with cultivating the audience by curating the right sort of song selection than it does with the aesthetic merits of the music itself, or even with the brilliance of the player’s presentation of that music. From the musician’s POV, this is, I think…well, it’s sort of disappointing. One wants to engage the audience in the music, not just using it as a tool, and it’s really quite frustrating to play gigs in which one feels like a glorified jukebox.
3. I realize that I’ve just dropped a big assumption in here — that the aesthetic value of a piece of music is not defined by audience engagement with it, or is somehow separable from how audiences respond to it. I don’t mean to argue the point right now, although I probably should. I think it’s reasonably safe to say, though, that the aesthetic value of a piece of music is at the very least not exclusively determined by how audiences respond to it. Audience response can, after all, change quite a bit over time, from region to region, heck, even from performance to performance. Properly curating/presenting a song and knowing one’s audience well can actually change how an audiences respond to it, as can external events entirely beyond one’s control (see, for example, Queen’s “I Want It All” used as an anti-apartheid protest song, an extension of Brian May’s notion of what he was writing about that he couldn’t really have predicted when he wrote the song).
[The photos in this post are from Short Notice’s fun show at the Cherokee, IA VFW for New Year’s Eve. See a few more in my Flickr feed!]