It's very nice for melee dps because of the auto-attacks as mentioned, despite what people say they contribute to a heft amount of damage, especially on mnk. I also use it in fights where there is a lot of moving around and I lose track of people, like in Crystal Tower. If i target them and lock on i can figure out where the heck they are then unlock.

It's useful to use if you can get used to un-locking when you need to move backwards but if you aren't a melee DPS then you're fine just disabling it entirely. Also an extra tip - for a good amount of enemies you can stand slightly farther back/slightly forward and give yourself some extra distance to run out of their AOEs. It might help you out while practicing using lockon if you choose to keep it available.