They collect Devilfruits, but all the Titanic Captains should by now have a fruit. What do they do with the surplus? They get more powerful, so they can gather more and more fruits. They could use them to enhance their weapons. Doc Q got a Mythical Zoan for his sick horse. Why not an additional one for his scyth? I am convinced that atleast Shiryu will get a Mythical Zoan (Kaido's) for his sword. Catarina Devon uses a sword too. Lafitte carries a cane with him. Blackbeard himself could put fruits into his pistols. So many possibilties.
