A couple of addenda to the metronome conversation that's still popping across the blogosphere. When I studied with Steve Lacy, he discouraged practicing scales with a metronome, but instead walking slowly, playing one note per step. (I think Steve talks about this, as well as many other things, in his book Findings, which I highly recommend. I find even seven years later, I still find a lot of Steve's voic in how I approach practicing.) I've modified this in my own practice to start at one note per step, then two notes, up until I can't pull it off, which is often 10, or 12, or 16 notes per, depending on the exercise and how adroit or rusty I am. I thought about this approach when I would play duos with Steve- when we played tunes, as opposed to free music. While the process was tremendously rewarding, and I always knew the tune better afterwards, it was very hard, I think partially because Steve's time was so personal and idiosyncratic, perhaps the product of literally walking to his own drummer. Friends said the same.
Second, when I was gigging a lot with No Sale Value in Boston, we would have various guests sit in, cellists, guitarists, dancers if we could, you name it. One night we had a rapper join us on a pretty straight up funk jam. Our drummer at the time, Jazon Nazary, had (and has) great time, but afterwards the MC complained that he had a tough time getting his flow going. He rapped primarily with tracks, almost always mechanized, which I suppose is the equivalent of rapping with a metronome. Playing with a live drummer, where the beat ebbed and flowed a little more organically, he found it hard to adjust.
David Ryshpen brings up issues of time and/vs. groove, which are fascinating, and I hope to follow up on.