I think the Ullman people would argue that you need to be able to move sideways (and stay vertical) as the boat isn't always going straight up and down. People who are more used to seats and bolsters often don't like this approach.
When I used to drive our previous Scorpion with the curved backrests, I would sometimes get a dig in the ribs when we landed at an angle. Personally, I'm not sure that a combination of a moving seat and a fixed bolster is the way to go. I know some who are trying this approach for the Round Britain Race.
Several have commented on the lack of side support and we feel okay using foot straps. The area when we need to be carefully is if we dig into the back of a wave and then turn. It might to be easy to slide forwards off the seat.
Anyway by the end of June I might have an answer! See you in Inverness. Gavin