DJ's take the job of *some* live bands because too many live bands suck. Too loud, no groove / poor song choices, dress awful, unreliable, etc. etc. and indeed etc. DJ's also cost less, take up much less room, and can play anything an audience might want to hear as opposed to "sorry, we've just been playing these same 30 songs for two years and that's all we can play." I mean in general, of course. So too when I write...
I've been in bands using backing tracks. It can sound good but I despise it. Often sucks the life and fun out of the performance, and getting the levels right from night to night and room to room is very difficult. It also takes an unusually high level of competence from mainly the drummer and singer (presuming vocal backing tracks).
That said the standard rule-of-thumb applies; as long as you're not hurting anyone then do what you want and have fun.