Jon - As a green fiddle player I couldn't play with a metronome (now I can) -- I just couldn't concentrate on both sounds enough to tell where I was in relation to its time. A guitarist who knows the tune will change chords at the right place to give you a clue. Also a metronome won't say "You're speeding in that passage !!" like a good guitarist will. Embarassment is a great teaching incentive !