If you don't see a point or profit to it, then don't use them, but I personally use them even if the fleet has high maintenance as I always add war ships to it so I can wreck any enemy using the trade node during war time and get enormous benefit from the node (ex. recently went to war with Netherlands as France and I annihilated all their fleets with my automatically-patrolling trade fleets, my profits from English Channel node doubled with both the Dutch having lost most of their trade power and the trade fleets boosting it far beyond what the Dutch provinces give through buildings etc.)
I believe that the total investment you put into those fleets - making the ships, replacing losses, upgrading them, is likely way greater than what you get in return from the increased trade profits, but the monthly income addition is what matters the most, in my opinion, not even the long-term gain, since it allows you to quicker stockpile money and allows positive events to give you enormous cash boosts, although many events also make you lose a lot of money then, since they're based on monthly income. I just see anything that increases my monthly income and allows me to handle a powerful army and fleet during wartime with no losses as very, very good, regardless if the methods are very expensive in long-term. You won't always have stockpiled cash to spend, especially if you're going on a deficit due to war, mercenaries, army and fleet (or ex. want to subsidize potential allies/enemies of your enemies, or just want to help a rival get back on its feet to prevent another rival taking its place or you're just so powerful you need to subsidize your rivals in their tough times so they don't collapse and end the challenge).