I would disagree with people. If you are checked in, with a seat assignment and your bags have been loaded on the plane then this will happen:

1) Gate agent boards the plane
2) Gate agent pages you when you don't board and checks to see that you should be in the airport as it is a connection and you have bags on board.
3) Boarding time comes and goes and plane is already delayed - Pilot announces they are waiting for a tardy passenger.
4) UA then begins unloading the hold to find your baggage - Pilot announces they will be on the ground for several minutes to unload your baggage - Passengers start complaining to each other that whoever it is that didn't board is an jerk.
5) 10-20 min later they find your bags, do a thorough screen for explosives and allow the plane to leave the gate.
6) The plane has missed its departure window and has to wait for the next available slot.
7) 100+ people are 20-45 min late getting to their destination, time and money is wasted unloading your baggage and everyone hates you all because you wanted to spend some extra time with a friend and abuse a rule designed to help people who are really have a reason for not making the plane.

IMHO, either A) Don't re-check your luggage and i) take it with you or ii) leave it in a locker or something at the airport, B) Get to the airport on time, C) have your friend meet you at/near the airport if you need to keep your baggage checked and don't plan on being able to make it in to the city and back on time. Take a second to think about how many people your actions affect and if maybe you should slightly inconvenience yourself rather than seriously inconvenience 100-200 people (passengers, ground staff, UA staff, Airport scheduling).