As a California cop for almost thirty years I got to see more than my share of knife attacks and knife victims. Most of the surviving victims reported that they didn't know a knife was involved in the altercation and that they thought they had been punched. None reported being confronted with a knife ala "West Side Story". The same doesn't hold true for the robbery victims I interviewed who were confronted with a knife. Usually the suspect displayed the knife and the victim gave up the property.
I was only directly attacked by a knife once and managed to call for help and keep out of the poor drunk's way. While I was legally and ethically justified in taking his life with my firearm, we eventually chose to mace him and knock the knife away with a baton blow to the forearm. It all worked out well in that case.
A friend, another cop with impressive credentials and common sense was confronted at an ATM as she withdrew $40.00 in cash. She was yondan in TKD, an MP Captain in the National Guard, and a crack shot who was armed at the time. Her assailant threatened her with a screwdriver and she chose to give up the money and get a good description and direction of travel which resulted in his almost immediate arrest. She received some criticism from her peers who suggested that she was a big sissy for not fighting or shooting the suspect. She smiled and reminded them that she was there and they weren't.
Aikido is an excellent tool in the event of a knife attack; almost as good as fleeing, passing out, or shooting your attacker. It is better than nothing, but I honestly don't believe that defending barehanded is generally wise and each individual has to make a choice at that instant - and further, none of us has room to criticize the choice made by the victim. As I said, aikido is an excellent tool, but there is also an excellent chance that some loaded knucklehead will seriously injure you, even if you win the encounter. You certainly will have a better chance than Joe Lunchbox if you choose to fight.
Sort of a case of "Artillery lends dignity to what would otherwise be a vulgar brawl". Train hard and avoid the situation if possible.