Sorry - but the whole karaoke analogy is in my opinion, tosh.

If a singer has a 5 piece band behind them, does it make them any better or worse than a singer with a Recording of a 5 piece band behind them?

Have you thought that maybe the vocalist singing to a backing track is:
a: getting paid as much as if they had a keyboard player/band with him/her. (how many of you dropped your bands in the 70s and brought an organ with rhythms because you could get more money as a solo artist?)

b: can't get a keyboard player/band up to the standard of the backing track. (maybe the vocalists are having the same trouble you as a keyboard player are)

c: just enjoy working on their own. Just as some of you guys do.

I know a few amazing vocalists that sometimes sing with a band or guitarist and sometimes use their own, or commercial backing tracks (Big band/ Swing).

Also....I know a few bands who reinforce their sound with backing tracks, pre-prepared by themselves. For instance the female vocalist of a local Showband sings over her own voice which is her backing singer. It also has the click track which runs through their in ear monitors. They are already a five piece and play small venues. (If they employed more vocalists, a third guitarist and second keyboard player it wouldn't even cover their fuel costs)

At the end of the day, the audience is there to be entertained and so use every means available to make sure the crowd get there moneys worth.

The only exception are 'professional audiences' (e.g. jazz clubs)who pay to experience the unique interaction and inspiration that happens between good musicians playing together.

As long as nobody is faking/miming its all good stuff as far as I am concerned.