I agree that there are a few problems if they have you board and then come and get you:

1. if the overhead bins are full, the crew will want to take your bag outside and stick it in the plane. They won't want it sitting in an aisle until the last possible moment.

2. once you are on the plane and they come and get you, you have ALL the negotiating power. Since you are holding onto your boarding pass and are on the plane, you can unreasonably delay them by wanting to negotiate while you are in your seat.

I've volunteered and been told to get on the plane anyway, without settling on a $$$ amount. If they don't want to lose their place in line for takeoff, they will get to see how greedy I am (the number I usually have in mind is the full-fare walkup price). Or, they could physically throw me off the plane, which, I suppose, they are low enough to do.