More seats means more people, which means more weight, and possibly more requirements for emergency escape given that number of people. Bunks mean just shifting existing people/weight to different areas, and even then only temporarily. During an emergency if there's time they could try to shift some number of people back to seats (some emergencies play out over many minutes, not just seconds).

All speculation, of course.

I would assume it would not be allowed for takeoff/landing (no emergency exits) which would mean no ticketed seats.