Danhole: Depends on the type of metal. I was in a death metal band and we had a few breakdowns and in the open space, we did harmonized sweeps. It's just used as a way to play arpeggio's at super fast speed. It's more efficient than picking every note and just has more finesse.
Watch this video to see how it's used (by the master himself) in a musical and melodic sense:
http://www.youtube.com/watch?v=p37s2RGVCxk
(Namely :50-:55)
This video is how it's mainly used in metal. Just a sweeping "melody" to go over the chug chugs, or as a solo:
http://www.youtube.com/watch?v=2u9KDMWJsAM
(starting at 1:21)
Franck Hermanny is the guy to watch on the bass though:
http://www.youtube.com/watch?v=NRNMwNgYqvI
Gawd: Yeah the guys who used the 1 finger per string thing is more like raking, it's plausible.. I'm just more comfortable with the way I play it =P
And yeah, I tried to learn a few times before, but once I started looking at it as a guitarist and followed Gambale teachings, it came very easily. You might benefit from looking up some of his articles and such from magazines.