I have a '77 Monark 1648 with the center seat removed, and it has never shown any loss of rigidity what so ever.
When I bought the boat the OO had un-riveted the seat and bolted it to a piece of plywood that fit between the ribs so it could be removed if needed. I removed it the day I brought it home, stored it in the workshop for a few years and finally threw it away. I run a 25HP Evinrude and it will scoot along at 21 MPH on smooth water fully loaded, with no twist, wobble, sway, chatter or any thing else.