I think it's important to be able to articulate to yourself why a knife on the person is a good idea. It may make you more proficient with it and imaginitive with its applications.

Something like having the basic sharpened wedge of forged steel, though, strikes me as more of a question of "why not" rather than "why." With the way everyone pats about themselves to make sure they have the mind-bendingly unimportant minutia of the wallet, keys, the enormous tangle of useless items I typically see in purses, etc., how can we not have the same number of and hopefully better proofs that having a knife on hand is a good idea? Don't let the complexities of a red-taped record-keeping culture blind you to the basic needs of a human. Can your wife really give you more real life examples of how she will need to whip out that extra makeup compact and social security card than you can for using a sharpened piece of metal? How deeper can you delve in terms of basic human needs or preparedness?

Not that this definitely addresses your specific issue, but I remember getting in huge trouble with my parents when I was 14 and they found I was carrying a lighter; they were sure I was smoking, but I just had it in case I needed to start a fire. I had this same argument with them, and I think it is applicable to your knife deal:

It's a very valid point to say "why would you ever have to start a fire?" However, I think that it's even MORE valid, and therefore eradicates the need for specific examples, to say "if I ever DID need to, imagine how hard it would be without one."