Seeing as to how bands at the national and club level have been using tape, clicks, keyboard sequences (is an arpeggiator 'cheating,' or just saving the keyboard player's hand?), on up to minidisk, samplers, and iPod tracks, since the 1970's, I'd love to hear you elucidate on how this "catches up" with them, other than to possibly expand their repertoire without hiring extra musicians, thus making them more hirable, and more fun to listen to.
Please note, I'm talking about people fleshing out their sound to perform songs they don't have the instrumentation to, otherwise; I'm not talking about doubling pre-recorded parts (which happens, but I don't see often) or flat-out pretending to play parts they're not. I'm not even talking about pre-recording background vocals, although I don't see a problem with that, either. I'm talking about replacing percussion and horn sections, and synth or piano parts with a sequence to a click.
To the OP--obviously a lot of people here have opinions regarding backing tracks. I'd say at least half, if not more, cover bands who play regionally, nationally, or in a couple of cases I'm aware of, internationally at $10,000 a pop, use backing tracks for various reasons. They put on amazing shows, crowds love them, and that's their job; they don't have to tear down after 45 minutes and go home early so they can wake up and go to their real job. Some of these people are booked solid a year in advance.
Obviously, whether or not it's appropriate to your situation depends on, well, your situation, but it's a standard thing to do in a cover band, and has been at all levels for decades. If you feel it's robbing something from your band's performance, convince them not to do it, or find another gig, but don't worry about whether or not it's standard or acceptable to audiences; it is.
Even for background vocals; if for no other reason that so many modern background vocals are heavily processed, compressed, vocoder'ed, auto-tuned (for tone a la Cher, not pitch correction), etc, and it's impractical to tour that way live, unless you're the much-mentioned Britney. So people pre-record their background vocals.
I wouldn't want to do it to every tune, but on some of our Journey set or Queen covers, I think it'd be completely appropriate, since it's difficult to get some of our band members to sing multiple-part harmonies consistently, or for that matter, to keep the drums from blaring through the drummer's vocal mike on a regular basis, which makes the whole mix sound bad.
YMMV, but don't let a bunch of people with ideas of what proper music is try to tell you it's cheating, somehow, or that it's not a live performance anymore.