There’ a lot of superficial knowledge involved in these and similar discussions.
For actual damage calculations look here: [Damage Guide] The big formula, modifier types, and how they interact!
Every skill scales with stats proportionally regardless of class, meaning there won’t be any useless skills. Most wizard skills recieve the same damage boost from INT as swordmen skills do from STR. So in general and with regard to damage, the LVL isn’t really important - i.e. a mage with 200 INT recieves the same damage boost as a dragoon with 200 STR does.
Of course, you can play around with damage modifiers (magic amplification, base damage modifiers, etc.) but - again, in general - the main difference seems to be the gameplay. All things considered there is no useless class or skill, you just have to decide which gameplay you prefer.