I think the musicians should be on stage.
There are circumstances where it's OK to use backing tracks, especially for studio-only sections or samples.
Some bands use pedals, pads, or other widgets to trigger sounds (I even had a siren pedal for one song we did in our school band).
I've got a couple of synth pedals (eg. the EH "9" series) which can fill out the sound, but might not be obvious as being played.
It's OK to have backing tracks in videos which are sync'd with the music.
And then you have the solo singers who use a backing track.
I can't be doing with miming though!