"You can't fight what you can't see" is a common taunt used by characters that are invisible, or otherwise unable to be seen (too small, fast, distant, etc.). It could apply to a battle in the dark as well...but only if the one making the taunt has night vision or something. A battle where no one can see each other would basically involved a whole lot of flailing around and hoping for a lucky hit. That said, in the games, the darkness doesn't have any effect on battles, just when you're walking around. Convenient, yes. Logical, no.