I am no trader but if I were, I would not want my fleets sitting in port full of cargo just waiting for some pirate to come along and loot the lot!
You would have to gain danger waiting in port for space in your warehouse just as if warehouse is full now or as not recalling a fishing fleet properly.
i dont know if your warehouses are maxed capacity but can upgrade them. Probably cheaper than losing income having your fleets sitting in port.
On second thoughts... I actually think it is a great idea to fill your fleets with gold bars and have them just sit in port.