If a soldier can't adapt to swinging at a goblin's legs to swinging at an elf's legs, then he deserves to be magma bathed slowly.
I don't think sword-to-sword combat is that simple. Learning how to swing a sword is different than learning how to kill a particular species. Like, an Elf might try to move around, dodge and feint, making your attacks more likely to miss if you aren't familiar with Elvish fighting techniques.
Now it sometimes shows in adventure mode when a creature is about to attack you, so you can parry or dodge. Maybe having information about a species would increase the chance of learning what a creature intends to do. Like "seems careful, likely to attempt to dodge/parry next attack" or "will aim for your neck" if you are really good at that creature's skill.
I mean, I got attacked by dogs in real-life several times and I can usually predict what they will do next by reading their body language, and I know what kind of stances and movements to take to both protect myself in case they attack and to intimidate the pack into leaving me alone. Also a few tricks to divert the attention away from myself and seem less threatening.
Then again, having a skill for every species in the game sounds bloat-y. Maybe only for dragons, hydras, and such?