That sounds oversimplified to me.
With no differentiation between even physical and magical attacks, then you only get one possible debuff, a flat damage vulnerability debuff that a job either has or does not have.
That would create a clear 2 class system.
You can bet that these won't stack either, that would be op, so they would likely all be the same amount (10%?) And it would be a case of any jobs that have this debuff cycling through, coordinating with each other's cooldowns in order to maintain this constant 10% damage increase.