The actual seatbelt will keep you in your seat, but it is fabric it wont keep you rigid, it is also slightly higher so exerts force at a higher point to keep you upright, apart from having a wall between people it is the best solution.
A lot of time effort and money is spent on research in aviation safety when given a reason why do passengers carry on questioning it's validity? Do as your told, when asked to do something it's not to annoy you or make your day awkward/unpleasant it's to save your life!!!!