Watch the Sweep Picking online guitar lesson by Jeff Scheetz from Rock Tricks
This technique also comes from a desire to go fast! Well, actually, that and the concept of minimizing your motion come into play here as well. But when your left hand is once again faster than your right, and your alternate picking can't keep up, that is when using several down or several up strokes in a row will speed things up.
Lots of quick players use this technique like Paul Gilbert and Frank Gambale. Once you get the synchronization of your two hands down, you can do some licks you have only heard in your head!