It is not stiffening the side rails, but a structural brace that fits into the area where the panels are on each side of the doors. If you look at later pics of 716/18s, you'll see additional braces on the rear rollbar to the rear panels to give additional support to the rear rollbar. This why all Pinzes were fitted with cargo screens, besides keeping loose items away from the driver,it also adds quite a bit of structural support laterally ( as much as the screens and bolts seem light for this purpose).
The disadvantage to a full width tailgate is that you still need to still brace the side rails in a rollover ( return channels on the ends of the tailgate could be used to hold the rails "captive", but I see many future problems with this). 2) The tailgate is restricted in it's swing with the pintle is still in place, and 3) tailgates are a good way for injuries since heavy items must be lifted beyond the tailgate and set into the bed, causing possible back problems . ( I had thrown my back out 35 years ago just setting a 2 x 3 mirror ( 20lbs.) mirror onto a glass truck.
IMO, the original door and side rails are generally the best way to go for most people. Pinzgauer still used the basic design up through the 716/18s, opting for side fold down rail, allowing for more bracing in the rear corners, and easier side loading ( If mine wasn't a 5 door platform, I would have had these years ago)

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The Trojan Horse... the 1st Pinz used to covertly carry troops into battle .
ATL Pinzgauer XM 718K TUM(HD) 6x6 FFR (aka The Green Grail)