I can say why I do it.

Psychology. Focus on that guy's rear wheel and devote your entire being to staying in contact with it. It's a motivator.

Also, when you crest the hill, if there's a flat, you're right there ready to get back in to the draft.

That said, if you don't have a sort of mutual agreement with the guy in front you're pretty much on your own. I'd never be angry if some random guy I'm drafting doesn't ride smoothly or decides to stop unexpectedly. That's my fault for putting my fate in his hands. It's kinda rude to expect him to accomodate my needs as a wheel sucker.