Actually I would say that knife is not a potentially good weapon for an aikidoka because knife is a better offensive than a defensive weapon and if you are not REALLY familiar with the knife it can turn against you. I guess spray gas makes a pretty good atemi
Anyhow knife makes a good threatening so you may be able to AVOID conflicts. Dangerous though...
P.S. I'd prefer a stick or a staff... Although I have a knife as well I NEVER carry it for example when going to a party (and potentially to drink something) Just sometimes when I know I'm sober and I have to go through dangerous areas. Or just cut some bread with it.