Unfortunately the amount of fuel burned in carrying extra fuel does mount up, particularly for larger operators. However it’s a safety issue and worth spending money on. I use my judgment to arrive with a sensible amount of fuel taking the conditions into account.
Going to India in the wet season, I’ve got fuel for the farthest alternate and about half an hours holding at my destination, if I can’t get in after a couple of goes I can choose the best option.
Flight plan fuel is simple the legal minimum to depart with.