Tks, once again for the answers....
well I have an SIB an OB with 360, as you all can see in this pictures,,, RIBnet Forums - Antonio's Album: JOMA on the Run
and here,,, http://vimeo.com/110067017
and the reason that lemme to propose such question, was beside watching you guys using yours SIbs without the sets,, that with my other SIb the plastimo 300 that you see on the video, with hard floor, for 15 days I gave him an intensive use of 8 hours day and fell more secure with the seats on... he din't flex so much.
with this new OB 360,,, I only experiment him with the seats on and with us four,,, and it plane, even against north wind and some 70cm chop.... when steering him I fell more secure on the seat that on the tube.
regarding the stress that the seats put on the tubes,,, well,,, that concerns me,,, but,, :P
once again tks..