Playing tunes with metronome is way more challenging than playing with a backing track. You have to keep track of the harmony and be more aware of melodic line to signify the changes in harmony. Great way to practice. Plus it gives you the pleasure of being solely responsible for the improvisation and not relying on someone else's backing track, computer etc...Nice idea.. will definitely do that from now on... All the Giants practiced like that. In fact here is a great soundclip of Coltrane practising what seems to be a tap dancer (or could be a drummer without his set):Backing track isn't there to merely keep time. A metronome would do that.