I will begin this suggestion with a quote from Sun Tzu:
It is said that if you know your enemies and know yourself, you will not be imperiled in a hundred battles; if you do not know your enemies but do know yourself, you will win one and lose one; if you do not know your enemies nor yourself, you will be imperiled in every single battle.(Thanks to NW_Kohaku for the correct quote)And another from Napoleon:
Never fight one enemy for too long, or you will teach him all your tricks of war. I was thinking about this the other day in 31.08 as I was preparing my 100% axedwarf army. Back in 40d it was my 100% marksdwarf army, and now I hear the next flavor is hammers. There always seems to be a flavor of the month when it comes to weapons, and there will always be some weapon that edges out the others when people get to testing and measuring stats and so forth.
One thing that could counteract such forces is the idea that you become familiar with what you are most often faced with. The more you go for 'the best' weapon combo, the better your enemies become at fighting against you, making it no longer the best.
So I propose that for every weapon skill, there is a corresponding skill 'against' that weapon, that you gain from fighting or sparring against that weapon type. I want to emphasize though, that this would not simply be a defensive skill, but also create opportunities for offense against weapon users of that type, that could have different types of entries into more varied combat reports.
This would also encourage players to train all weapon types, so that they can spar against each other and be prepared for whatever they face, but also have more diversity in their attacks, should their enemies train themselves against any preferences that you exhibit. I'm thinking that goblins could study you and learn how often they face this or that weapon combo.
The more you mix it up, the better you will be on offense, and also you will be better on defense from more varied sparring opportunities. It might even be a reason to train military dwarves in multiple weapons, choosing the weapon you feel that your enemies have seen less of.