Panpiper, don't forget that there is a cap on how much fuel your bases can store, once they reach the max they stop mining and it sits idle. If your fleets fuel storage is greater than your base fuel storage, or more importantly your bases fuel storage NEAR the front line, then when it comes to refueling your ships you may find that you can't refuel them all at once at a convenient place.
Also, with more fuel tanks they will be able to be in the field longer, which at the start of a war can make a huge difference, but it will take longer to refuel as you either have to a) travel further afield for fuel or b) await stations gaining the fuel. I've had it before where most of my fuel reserves were on the wrong side of my empire in the early game, and I ended up having to suffer a few systems getting demolished whilst my fleet travelled to a fuel station with no fuel then back again. I've learned since then!
Late game I tend to set up quite a few resupply ships near the front line as I have about 4-5 fuel tanks on my destroyers that are the mainstay of my fleet, normally group my fleets in 12's and 16's and two or three fleets (36-48 ships) all refueling at the same time will strip a couple of gas mining stations with 20 cargo bays full of caslon/hydrogen.
This is one of the reasons I love DW so much. No matter how long you've been playing, you can play a game and something goes wrong and it makes you think "oh, I never thought of that!". Just goes to show that the game is so incredibly complex, yet it's well designed that sometimes these things go completely unoticed!