There is a valid reason for having OCPD in both legs. I have copied SolarPro's image and placed two ground faults on it, since as we know we have to have at least two ground faults in an ungrounded system to have ground fault current flow. If one of those faults is after the point where the fused conductors are combined and the other fault is on one of the unfused conductors going into the combiner box to be combined you will have the full combiner output current, minus one string, flowing in an undersized conductor, as shown by the thick line between the fault and the combining bus. There will be no fuse to stop the fault current, the conductor will most likely overheat and possibly cause a fire.
If there were fuses in both legs in this example the fuse on the faulted negative string conductor, located between the fault and the combining bus, would open and the fault current would only be the current from one string since there would be no return path for the other string currents. You can show this same issue using any combiner box in an ungrounded system with fuses on only one leg.
While some may say this combination of faults is unlikely the fact that it would cause a great deal of damage if it happened leads me to think it's better to play it safe than to be sorry, so put in the extra fuses.
If there were fuses in both legs in this example the fuse on the faulted negative string conductor, located between the fault and the combining bus, would open and the fault current would only be the current from one string since there would be no return path for the other string currents. You can show this same issue using any combiner box in an ungrounded system with fuses on only one leg.
While some may say this combination of faults is unlikely the fact that it would cause a great deal of damage if it happened leads me to think it's better to play it safe than to be sorry, so put in the extra fuses.