I mean something like the proficiency system used in Mount&Blade.
It could make players to have more consideration before changing their mercenaries’ weapon. And it could also help to build closer bond between a mercenary and their weapon, which could be better for role play.
In the earlier part of games, I never want to commit most mercenaries to a particular weapon, and (even if I do think a particular weapon would be ideal for one of them) the ideal weapon is often not yet available in high-grade form, whereas other weapons that aren’t quite so fitting may be available in abundance at good/high grades.