"Was the information useful?" I'd say yes. If you identify the presence of a weapon before it comes into play, it's easier to keep it out of play than to remove it from play once it's been assessed. That applies to good guys and bad guys. Bad guys who spot knifeclips are, generally, not going to be disuaded because of the knifes presence, they will simply make sure that you don't get to bring it into play. Bad guys make their decisions on who to attack based on lots of things (body language, eye contact, awareness level of the intended victim) but rarely on the presence, or lack thereof, of weapons.

The problem with a uniformed cop carrying a knife and gun, both plainly visible to the bad guy, is that should the bad guy choose to attempt to disarm the officer, the officer's attention must be split between protecting two weapons. There have been cases where the bad guy spotted both weapons, made a feint for the gun and accessed the officer's knife.

The role of the knife, defensively speaking, is not to allow you to say "get back or I'll cut you" but rather to say "get back or I'll cut you, again". The knife is more useful to you if its presence is not known to the bad guy until it is in use.

Of course, carrying a knife, as a defensive option, without training is not advisable. The knife is every bit as much a lethal tool as the gun and subject to the same restrictions on use.

Gomez