I've just started to learn to sweep a couple of months ago, and it is an INVALUABLE technique.
To your questions. Sweeping can give you a completely different more "fluid" sound than
alternate picking. Changing pickup position and using taps and slides in sweep gives you an unlimited amount of sounds.
No way is sweeping only good for fast playing. In fact SLOW playing sweeps are much harder and just sound incredible when played right. Some of the best sweeping I've heard has been slow, fluid
neck position sweeping with slides in it. Some of the worse is Angelo's insane blurs of notes sweeping.
To sum up. Definitely learn sweeping, and stick to 3 string sweeps until you've mastered them then move on. You can use them to blurt out an insane 100 notes or you can make them sound beautiful and fluid.
Just my $0.02