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.
Most always, the TSA has missed this during "random secondaries at the gate". The rare occasions where someone has been caught has generally involved no "world of hurt" whatsoever for the person "caught".