A happy consequence of us making each rail along the uprights both full-length and externally-mounted is that they can be removed so as to form the basis of the seat, which in turn will stiffen the craft along its length.
A catamaran sank in rough weather the other day north of Cyprus, and they are by no means a silver bullet. Trimarans are considered a better compromise altogether, but then they're a lot harder to design and build.
The principal weakness of the cat is the bending forces applied to the front-end of each pontoon, as the farther these are apart the greater the twisting force that they apply to the centre-section or bridge adjoining them.
Do you remember Pete Goss? No, I don't either but one of the prongs of the boat he was sailing snapped off in this way despite being made of carbon fibre:

