Contingency fuel is misnamed. It is a small amount and may be burned before takeoff, unlike trip fuel and reserves.
There are days when it might actually be a perfectly reasonable option to burn all the calculated/documented contingency fuel on taxi-out, even some of your trip fuel, but nevertheless get airborne. Then on other days it might well be more sensible to return to the gate for a top up...but that decision would be made as a result of discussion of options with my colleagues on the flight deck/operations, hopefully a touch of CRM, and also I'd throw in a bit of old fashioned airmanship, as in "is what I am suggesting safe"? I'd really rather not have yet another process codified by the lawyers, e.g. "if all the flight planned contingency fuel is used before take off you must not get airborne".....
is everyone happy with the legal fuel regs?
Personally: Longhaul arena, always have alternate planned. Load taxi fuel plus trip fuel, plus alternate fuel, plus 30 minute reserve, plus
statistically derived contingency fuel, normally to the 95% level,...plus any extras demanded by things such as ETOPS,
plus the option to load on any extra I want ( which I sometimes do, often don't)...I happy with the fuel regs I operate under? Yes.