This is not a settled question in the world of marine solar systems. The reasoning you present, the panels produce power even with the controller disconnected is sound, especially in high voltage high current systems. In smaller systems with perhaps one panel it may not be necessary. If one is installed, it need only protect the positive side, a double breaker does not add any additional protection. I did not install any circuit protection between the panels and controller.
There is considerably less controversy about circuit protection between the DC+ bus bar or battery and the controller. ABYC is quite clear that all circuits connected to an unregulated power source have circuit protection sized to protect the smallest wire downstream from the circuit protection device (fuse or circuit breakers). A battery is an unregulated power source because in a direct short between DC- and DC+ the full capacity of the battery can pass through the short, which can be hundreds of amps. Alternators, solar panels, and battery chargers are regulated power sources. The amount of current they can provide in a direct short is limited by the capacity of the device. A 100 watt solar panel can not produce more than 100 watts which is about 4-5 amps @ 20-25v. Cabling can be used that can handle that load even in a direct short.
Perhaps the strongest reason to including a switch of some sort on the DC+ from the panel is for servicing the controller. Some controllers, like Victrons, have a specified order of connection, i.e., the controller must be powered up and connected to the battery (or DC+ bus) before receiving current from the panels. There a ways to accomplish this with blankets over the panel, disconnecting one of the panel leads, or waiting until nightfall. I"ve done all three. A switch would be more convenient.
