Why can't you learn about a weapon when you are fighting against it?

Seems like if you are fighting someone you could learn things by watching how someone else uses it.

that all classes can't learn to use all weapons, just by fighting people who already know that weapon.

e.g. if my invoker fights an axe-spec, should I start to learn axe skills?

Answer: Nope, because of game balance. If my voker could learn every weapon, it would probably make him over-powered because effectively a weapon you can't parry is a kind of vuln.

Well granted some professions aren't proficient at all weapons

what would an invoker need an axe for anyway?

Maybe make it a warrior only thing...since they are weapon masters and can learn ALL weapons.
And make it so they can only learn it to a certain % (say 85%) since you can only learn so much from watching and the rest by doing.
*shrug* just a thought

Because, by learning to use an axe, you can parry axes better.

You need to have a basic understanding of how it works already, in order to learn through combat by watcing a foe.