Yosharian
Arcane
Mercs cost money to hire. You might have lost a story advisor due to no fault of your own. Most of the story advisors have pretty strong main stats. Kingdom Management DCs can get ridiculously high. Merc advisors are already penalised because they lack the personality of the story advisors.The stat malus kind of makes sense since you can easily min-max the shit out of the important stats but then again, kingdom management isn't that relevant.
So no, it doesn't make fucking sense. It's a single-player game, if I use mercs and I end up gaining a very slight mechanical advantage (and not really, because as I already said, the story advisors have training projects), then that's my decision.