I've mentioned this before in passing, but I wanted to bring some attention to it in today's post. In aikido (or judo or karate or etc...) we mostly practice empty-handed, but occasionally put a simulated knife in the hand of the attacker. It it not that we want to emphasize those ridiculous "knife defenses" that are the bread and butter of so many martial arts classes - so why do we even mess with a knife?
Simply put, the addition of a knife highlights a particular weakness in the defender's mindset. That is, if the defender suddenly becomes sharper, more alert, quicker, and more precise when a knife is thrown into the mix, then he was under-estimating the potential of the unarmed attacker.
As you practice aikido, you absolutely have to treat every single uke as if they are the most dangerous guy on the planet and they have a knife.
So the knife is really just a measuring device that points out this partcular suki (weakness). The goal would be to train enough that knife changes nothing.