I think to remember an incident where the non-linked sidesticks nearly
lead to a catastrophe. I don't remember it exactly, but an A320 was about to land (or had already touched down) in Sydney when the PNF realized that an aircraft that had landend on the crossing rwy would not come to a stop before the rwy crossing.
He started pulling the stick while the PF who had not anticipated the other aircraft continued pushing it such that the signals from the sticks neutralized each other, since they are additive. They then realized the situation and went around. This would certainly not have happened if the sticks were mechanically linked.
Please don't shoot at me if I did not get the details right, I read about this somewhere several years ago.
I believe to have heard that after that Airbus built in a warning horn which is activated when the sticks receive controversial inputs. Anybody of you Airbus experts know more?