They don't call it "chess on wheels" for nothing. Usually, if there is a GC contender in the breakaway, then the other GC contenders will join the chase. In this years Tour, Rasmussen broke away in one of the early stages and the leaders let him go because he wasn't a contender. However, he gained a whole lot of time on the leaders and was definitely within sight of the top spots, so it probably wasn't such a good decision to let him go.

bluyak says that drafting is terribly difficult and dangerous, but it's really not so bad once you have enough experience. You don't actually sit and relentlessly watch the wheel in front of you. For that matter, if you did, you would certainly not be able to react in time. You watch the rider, and you just keep the wheel in your peripheral vision. And you trust the other riders around you to hold a very constant speed. It's certainly safer on a closed course with no debris and road hazards, than it is on the street.