Personally I think we are missing the mark here.
Use of contingency on ground and deciding whether to depart or top up should be based on the amount of contingency fuel you are dispatched with.
If I'm on a long flight, I will have either 3%, 5% or 20 mins worth of contingency. I have used some or most of that and I'm departing from a busy airport, I would definately consider departing and assessing my fuel situation en-route. In co-ordination with the company then decide if we do a tech stop at "not so busy" airport.
If I'm on a short flight and dispatched with 5mins contingency and departing from a "not so busy" airport, I will go back and top up again.
You just have to add up your situation and make a decision on what's the best on that day.
Making only one decision (no decision making) to return back to the gate to top up without considering the situation is not what you are paid to do!
Oz