Mavimao said:

Ion cannons were used on certain ships in the Roque Squadron games. They could momentarily disable certain things like laser cannons. in retrospect, they were actually pretty crappy weapons because it was easier to just hit whatever with actual lasers and freakin blow it up.

You had to use them in the Kessel mission to disable the train transporting Wedge to prison. I think that was the only mission they were available in, actually. It was kinda dumb. Fun to shoot TIEs with it and watch them fall out of the sky and hit the ground though.