First of all, I agree that knives are for cutting, with the exception of specialized knives meant for emergency or combat zone conditions that may need to perform on a different level. These knives are consequently rarely effective at the kind of precision cutting that a regular knife can handle, though.
I can even agree that you might carry a dedicated self-defense knife, and not use it for any other more mundane purpose. That wouldn't work for me, and I don't think it's good advice for most of us, either.
Ignoring for the moment the question of how effective a knife actually is for self-defense, most of us don't train for close quarter defense. The knife is a backup, for ultimate emergencies that we would prefer to face without resort to close-up deadly force.
Not training for that knife in that context, when the emergency arises, we might not even "think" to go for it. But I would reach for my EDC, for the knife I reach for whenever I have something to cut. Maybe it won't be razor sharp, but it won't be dull, either.
By the way, we've had many threads about the hazards of loaning any knife under any circumstances. My attitude is, if they don't have a knife of their own, they probably don't know how to use mine safely either.