Henry Smith Shihan teaches that if I am trapped in a position where I must practice tanto tori then my Aikido has already failed. I should have left the bar, or parked near the guard post or been already talking on the phone to the 911 dispatcher before the knife was drawn. This mindset, free of testosterone blindness, is the highest practice of O'Sensei's art.
Absolutely. It's always best to back away from a situation that may escalate beyond the usual fistfight. Where I live, fistfights can very easily lead to the introduction of edged weapons. If a situation starts getting tense, expect sharp and pointy things to be tossed into the mix.
It's irresponsible for a teacher of any system to delude his or her students that a knife attack can be easily defeated with the techniques they teach. "Testosterone blindness" + knife attack + unrealistic expectations re knife defense = bad, bad news for the defender
I consider PTK knife training as an education in respecting the weapon, knowing what it can do. By studying how to use it and the damage it can do, my students realize the risks and they lose any cockiness they might have with engaging an attacker with a knife while empty-handed. I always teach empty handed knife defense as a last ditch response to a worst case scenario.
Which is why I strongly recommend that aikidoka studying both the formal tanto dori waza they need to do in the dojo and defenses used against more realistic knife attacks. Practicing the standard knife defenses is ok; it's part of Aikido and if I recall correctly, required for some Aikikai yudansha exams. But I do hope that most Aikidoka look beyond the formal techniques and see how they can be applied against the typical street attack.