Fist let me say one more time for you Don. The majority of Aikido's techniques are designed for you to HAVE THE WEAPON. This means that YOU ARE THE ARMED GUY. So most of what Aikido teaches are not weapon defenses, but in actuality weapon use facilitators.
But for the sake of argument I'll argue Aikido as used to weapon defense system (which is the minority of it's teachings).
The techniques are not limited to knifes. Any thing held in the hand will do, it's just that knifes are likely the hardest to deal with in their range. If the day has gone by where people fight with hand held weapons, they yes, Aikido's techniques are useless. But I don't think we're there yet.
Watch any of these real life shows where people braking the law are on display. 80% of the time they come at the other guy with a weapon, they just pic up a rock, or a lamp, or what ever is handy and start swinging. They show a weapon in the hopes that the guy will give up his money, or what ever they want with out a struggle. These are the times that Aikido's techniques are useful.
Remember, as Michael Varin often says, assassinations are different then confrontations. This is to say that yes, people do plan to kill others. They plan out how to sneak up on them and stab them in the back. But there are also confrontations. A confrontation happens spur of the moment, when someone pulls a knife off the kitchen table and come at you. Or when someone simply wants your money, or to rape your wife. They pull out a weapon to scare you, in the hope that you won't struggle with them. It if far less common for someone to want to kill you then just humiliate you, or take what you have. You are less likely to run into an assassin then a mugger. Rapists are more common then serial killers. Confrontation is more common then assassination.
Really bad situations happen with weapons. If someone just wants to prove how tough they are (I call this ego fighting) they won't use a weapon cause it's cowardly. This is the type of fighting you will see in a bar, or on a school ground, or at a dance. These situations might hurt your ego if you lose a fight, but you won't die. However a bad situations is when someone wants to take your car, but you won't be able to go to work with out your car, and if you can't go to work you can't pay the rent. Someone trying to take your car, is likely going to use some sort of weapon to do it. This is when the techniques of Aikido are useful.
Jez, I hate writing long posts....
I started writing a long post to this about the futility of self defense training in my area based on crime statistics and personal experience. I'll probably start a new thread though, as it really deserves its own thread. But in a nut shell, the crime in my area really doesn't support a need for self defense, unless you are a child or housewife. The majority of the crime seems to be domestic violence, thefts. However I need to do some more research as the internet is giving me some conflicting stats.
But I don't carry a weapon. If I did, it would be my handgun. So better served by training to retain and use a handgun. This would require me actually strapping on a toy gun of some type, and having people try to take it away from me while I try to "shoot" them. Something not done in my aikido class. We usually work from wrist grabs, lunge punches, and overhead chops.