wATK is a huge deal for all mechanic damage skills, even those that use weapon weight/str/dex/vit in the formula.
Don't disregard that the pilebunker's gigantic base atk value gives a huge bonus for a high STR char, which I imagine all mech builds will be (unless you're a dedicated SD WoE alt.)
Vulcan Arm is a total missed opportunity to have a nice supporting skill.
If I was unilateral dictator I would do a few things. First, I'd change it (and nearly all other mech skills) to scale with base level at 1:1 instead of the current 5:6, and would change the formula to be: Atk % = (Skill Level * 100 atk) + (30 * Remodel Mainframe Level) + (Dex * 1).
But if I had such power, I'd first make it so Arm Cannon's AOE doesn't shrink as you use higher levels, as that's just a slap to the face.