EDIT: What Keith said.

No, drafting doesn't slow down the front rider, and in fact, should give them a slight benefit, because the other rider prevents the airstream moving over them from going turbulent behind them, which can cause low pressure areas that slow you down. In cycling it is a pretty subtle effect, but its noticable in other aerodynamic fields.

Psycholocially, it is a bit draining to have someone that close to you that you can barely see, and if the two people aren't trading off, it can be a little annoying. Drafting certainly isn't fair in time trial type of events, and can be dangerous in other cases, which is why some types of races forbid it.

peace,
sam