You want to keep your cue on the line of the shot, moving it from side to side, up and the down means that you need to bring it back on line before striking the the cue ball.
If you pull it straight back, you only need to push the cue straight through.
I doubt any of the pros made a concious effort to develop a "wiggly" stroke, it's just a natural motion for them.
I would suggest studying Buddy Hall as mentioned earlier, as well as the snooker players such Ronnie O'Sullivan or Neil Robertson (two of the best strokes out there)
Ronnie's stroke (start at 3:06), the three consecutive centuries he had (whilst alternating hands for no real reason) almost made me quit altogether.
http://www.youtube.com/watch?v=KmDPJDwcFDs&feature=related