use metronome with my students pretty often, mostly in these situations:
1) studen't can't play a riff cleanly, because he's trying to learn it at the original speed. This happens very often, almost always. Solution:
1a) find the original tempo and write it down, let's say it's 120
1b) set the metronome to say 50 and learn the riff (this step takes ca 80% of the time spent on the riff)
1c) when you can do it at least 3x in a row, raise the tempo to 60
1d) etc. till the original tempo.
2) student is fluent in playing in metronome, but his rhythm is too strict and dead, because he plays all notes at the same volume. Solution:
2a) show the student what the
metre is and how the natural accents are dealt with.
2b) play with metronome beating on 2nd and 4th beat only (example: song in BPM 100 -> metronome BPM 50)
3) student can't hold the tempo on his own, hence is not helping the band to have a steady rhythm. Solution:
3a) listen to the tempo for a while, then turn the metronome off
3b) play the etude
3c) listen to the tempo again and compare, whether you sped up.