I suspect that these frames do add some structural strength, but don't really know. I would not cut the bench structures out and leave them without reinstalling. I did have a bulkhead that supports the chain plate rot and fail (the things that look like book shelf supports). I cut it out with a grinder, including removal of the glass tabbing - obviously being very careful not to grind out any of the actual hull. I then made a new bulkhead (I used poplar, as I could not find marine ply in my area) and used epoxy plus filler to glue it to the hull and fillet the edge, and then expoxied glass cloth along the edge. I don't see why you could not do this with the benches, though it'd be a bigger job (though with less risk should the seam not be quite strong enough).