It is true, a fuse, switch, circuit breaker, undersized wire and even excessive connectors in the wiring to the controller will increase the resistance and reduce your total voltage at the controller.
I picked Klixon 2TC2 aircraft breakers because they have a low voltage drop, (about .25 volt at full current, less drop on lower currents)
I know any voltage drop is a red flag for solar but with panels that provide more voltage than the controller needs, it works for me and I can see 20 amps into the controller from 4 panels in good sun. (Yes it will be much less in poor sun)
This is page 3 of my write-up showing my estimated losses in my system
(Whoops, I just realized that I did not add in losses for the breakers in those figures)
My theory on the need for circuit breakers or fuses is; It is true that if one panel shorts it will not trip its own circuit breaker, it is the other panels tied in parallel to the shorted panel that will trip the breaker to the shorted one, allowing the remaining panels to keep supplying current.
If my 4 panels put out their nominal rated current of 5.25 amps each (going through my 7.5 amp breakers) for a total of 21 amps, and one of the panels shorts out, the breaker to that panel is going to see 15.75 amps from the remaining panels and its breaker will pop. If I had more panels without any breakers and exceeded the amp capacity of the wiring, the current coming from the good panels could burn wiring going back to the bad panel.
I may have over-engineered my system, but it works for me, and being able to pull a breaker allows me to isolate the panels one by one for maintenance or to see how they are performing.
That came in handy when 2 of my panels went bad. Vibrations from a road trip opened up some bad solder joints in the panel diode junction boxes.
I was able to determine which panels were bad inside the vehicle by pulling breakers and checking outputs of each panel. Then I pulled the bad panels and re-soldered the connection and I was back up and running in a day.