I've never actually done this, but I had an instructor once who was big on sword work. He studied Kendo (I believe it was Kendo, might have been something else) as well as Aikido.

He said a lot of things that stick in my mind years later, but one which I think applies:

"If you are facing a person with a sword, get used to the idea that you are going to get cut. If you can live with that, you have a chance of surviving. If getting cut is a problem for you, you're going to die."

That's probably not exactly how he said it, but it's pretty close, I think. And he said this many times. Sometimes it was a knife, rather than a sword.

I've thought about this a lot over the years when practicing weapons defenses. I'm by no means an expert in this area, but I believe he has a point.

I think Chris is right on. I assume everyone is familiar with Toby Threadgill's great essay on Assumptions?
http://www.aikidojournal.com/article?articleID=685

There is another similar story about Takamura sensei that is also told. In a similar situation to the one in the article Takamura pulled a knife on a BJJer who had him in the mount. People who saw it said it appeared as if the guy levitated off him.

What it comes down to is that aikido descends from arts employed by swordsman for use in a venue that assumes bladed weapons and multiple attackers. I think you need to know this information to do aikido successfully. If you impose different assumptions either your aikido will change its essential nature to fit those new assumptions or because of the miss-match your aikido will be inefficient and ineffective in the new venue, like driving a screw with a hammer.