Not necessarily. A decent band, even without a following, can keep punters on site. A number of venues get their bands to start and finish later - ideally as close to closing time as possible, because they know the venue starts emptying pretty rapidly when the live music finishes.
A pub makes a very decent [typically around 70%] gross margin on wet sales, and they can easily figure out whether it's worth their while putting on live music, or whether they're better off with a DJ or no music other than the JukeBox or their own playlists.