Even if you're attacking the Walker in Walker Assault you're better off with the cycler rifle. Cool down is much faster and if aimed correctly does considerable damage to the At-At (you have to hit it in the tube thingie on the bottom).
Only thing where Ion Torpedo has an advantage is attacking At-St's.
I'll give it a go.
I mostly carry the Ion Torpedo in case I find myself in one of those games where the other team has air superiority. That way I can at least disrupt some strafing runs, and get the odd kill if lucky.