Why do Airbus sidesticks add their inputs?
I believe that there is some logic in the design, actually, even if I don't really fancy it. If you think of the traditional yoke as a control element where a certain force (or moment, rather...) results in a certain change of flight path, then if both pilots apply the same force (rather than targeting the same deflection), the resulting aircraft reaction is more pronounced than if only one pilot applies that force, although probably not with linear relation. The sidestick controlled flight control system does the same.

What is missing much more, to my mind, is feedback to one pilot about what the other one is doing with the stick. I believe that having them interconnected or back driven would probably not have been such a bad idea.