Has anyone realized that you can't use the ion cannon to disable Moff Serdon's shuttle, the shuttle on level 3, nor can you capture enemy craft your self? The weapon is virtually useless in missions, except for the fact that it can be fired faster than the Y-wing's lasers, it can "lock on to" enemies that are not directly in front of you (as a turret follows its target), and when fired simultaneously with the blasters, makes a decent weapon that can take out enemies with the power of a laser cannon, and with the firing rate of the ion cannon (the blaster does most of the damage, and this is easier with a controller setting other than the default.)
About the "locking on", the A-wing can do this too, but only vertically; I noticed this in Liberation of Gerrard V (level 5).

I would say that you shouldn't even bother with the ion cannnon. I havn't found much of a use for it yet. So you shouldn't really bother with it.