If you think doing that gives you a blank check to add as much fuel as you want, I'm afraid you are mistaken. I wouldn't normally delay a flight to take off fuel, but if it was bumping revenue (or possibly even non-revs) I would have no qualms about telling station operations to have the plane defueled back to the planned gate fuel. If that upsets the captain, I'm happy to let him or her talk to a supervisor in the flight department about it. While the captain is having that discussion, I'll be sure to notify load planning not to send the final numbers with more than the planned gate fuel.
Honestly, as long as there is not a weight issue (or a computer issue on my end) I don't mind the crew adding fuel on board. However if it exceeds a certain amount, we are required to document the reason why, so replying "Because I'm the captain and that's how much fuel I want" is not an adequate answer in my book. I'll put down whatever you want to say, but I'd prefer it to be something other than "Jackass Captain" for the reason. If I feel it's a truly ridiculous amount, I will also document it in the shift log in case management comes calling. I don't do that often - probably less than once a year - but as long as the captain doesn't mind potentially getting a call about adding an extra hour's worth of fuel on a two hour flight when the destination is VFR, I'll go ahead and send the new paperwork.
True story: while it is rare, on more than one occasion I have had to turn down a captain's fuel add request because the crew was requesting an amount that would exceed the max capacity of the aircraft's fuel tanks.