Outdated Mining Robots. I love the magic attack of those guys, LOLSPARTADRILLU! It looks epicly lol but does hit quite hard if your frontline has low MDEF.
Anyway, back on topic about Cannoneer. Smoke Bomb never loses utility, it's value is the -ACC to mobs, which can be really useful. But Smoke Bomb aside, cannon normal attack is very weak compared to gun or bow, and doesn't hit flying. Before I got a 2nd B class merc, my formation was;
The way i run my lines, frontline is for melee seth, midline is for ranged seth. I found that my Cannoneer was relegated to sitting in the bottom right corner and doing nothing but Smoke Bomb. I found that in place of the Cannoneer, i could stick an Elementalist and get a LOT more utility out of her. Putting the Cannoneer in the midline was a bit of a waste as while sething him was good for the splash damage on trash mob groups, it was poor damage overall compared to a single shot from the Prophet.
That's just me though, my formation is very highly geared towards PvE and specifically BOSS KILLING. I actually don't do that well towards 3v1 trash mob groups. A Cannoneer (or any cannon type merc in general) is VERY good for use when fighting 3v1 as they have a high multi rate. Doesn't work well for me so meh, this is just my personal opinion.
Boomy's line is a very good 'all round' midline.