It did look like a nice way for FC owners to make a some extra money while doing the CG and I've no problem with that, they were in (most cases) providing a useful service and did have costs to pay, in terms of Tritium and maintenance costs.
Of the 29700 tons that I hauled altogether, about 16000 tons I bought (i.e. leeched) from open Carriers close to the CG.
In the previous Marlinist Refugee trade CG, I docked and bought CG commodities a few times from CMDR Ralph Vargr's Traveller Box and on one return trip I brought Tritium and topped his FC's Tritium level. Unfortunately there was no Tritium available at Spirit of Nysa so I couldn't 'pay' with that. That said, I sure could've brought a full hold of Tritium from the bubble and paid to FCs that way, wouldn't really have affected my CG ranking at the end.
I haven't really looked into Fleet Carriers myself, but how big of a bother it is to keep Tritium level topped up? Is it a big help if a random CMDR donates you some Tritium or is it more of a nice gesture?