Originally Posted by
Often1
The answer is that it is not allowed, is actually illegal and, at a minimum will subject you to a world of hurt if you are ever caught. For what it's worth, TSA does random secondaries at the gate, often checking ID's just to root this out. At that point, you're standing at the gate with a useless ticket and either purchasing a seat at walk-up fares or slinking away.
Odds of the TSA "random secondary" finding a valid boarding pass that matches the person's ID is about a million to one. If you don't know the method we're talking about you're in happy pretend land.