Well, right now I woudn't know what the advantages of a metronome really are because this whole conversation with Jeff has led me to question my relationship (which, I admit, was always rather snuggly) with the metronome. Have I improved because of it? Or have I improved at the same time I was using it because of the focus I was putting into improving, of which the use of a metronome was just a materialization?
Anyway, for me the metronome was NEVER about keeping time. My music theory teacher would always tell me to stop counting and 'go freestyle' at rhythm reading exam because she knew I would be getting it right.
It was more a way of smoothing out coordination. A muscle thing to ensure equality between the fingers. You know, the infamous triplet thing. Everything sounds perfect when you play it in eighths, and you try it in triplet and it's suddenly clear that one of your fingers is way weaker or clumsier than the others in a specific gesture. That's the kind of thing I used to sort out with the help of a metronome. It would underline muscular weakness and lack of coordination. Nothing to do at all with timing.
Now I can tell you why I would favor a metronome over a loop: it doesn't annoy me with a different tone of instrument than mine (or slight differences in tuning), doesn't imprint on my musical understanding of a line someone else's interpretation, doesn't force me to stop playing to stop the loop once I've completed the exercice, and so on.
That still doesn't mean it's good. I was musically brought up in a system where discussing it was not an option. I remember buying my first metronome when I was 4, all I got to pick was the color (I picked orange
). Then the teacher would indicate the tempos I was supposed to reach for next class. That was it.