On shorter flights, you still have the weight impact from that empty tank and its infra. If they had to make a bump out on the fuselage, you have a permanent aerodynamics hit.
It's like permanently attaching a cargo carrier to your vehicle to carry more fuel. Fewer fuel stops, but more burn per mile. Can make sense in some situations.
The study of breaking up long-haul flights for reduced fuel burn is usually called: "Intermediate Stop Operations"