I bought but have yet to add plates for the inboard side of the rear crossbar as I think its worthwhile protection.
The upgraded plates can be added to the inboard rear crossbar locations, but they're not as easy to install as the other locations. The recess in the hull is not as large as the other spots so it's a very tight fit. Before putting them on my boat, I put the channel portion of the plate in a bench vice to squeeze it closed a little bit so it would fit in the recess more easily. Also, the deck flange may block the two outboard holes of the bracket making it difficult to insert the 1/4" machine screws through the bracket and into the hull - the head of the screw is blocked by the deck lip. My solution to this was to use a dremel tool with sanding drum attachment and grind two little half-moons into the flange until the screw head would fit past the deck lip.