Most people could do with a single knife with a smallish blade, but it's the emergency use that many consider. I know of one guy who pried open a theater door during a fire alarm using a tanto blade. His partner went back to the patrol car to get some tools, but this guy used his folding knife to pry the door open. True, a duty knife is expected to be strong and dependable, but no one knows when an emergency is likely to come up.
Another man cut a woman free when her scarf got caught in an escalator. The scarf was choking her and she most likely would have died had not he been able to cut her free. Knives are one of the oldest tools of man, and they still can come in handy on occasion. Me, I got turned in for having a knife at work. One of the secretaries couldn't open a very large flat package that was taped up. I opened it with my knife, which was just a 3-inch CS Voyager, and later that day I got a call from my boss wanting to see me. She said one of the secretaries had become alarmed by the knife and she asked why I had it. I reminded her that I had used it to open a package. I also told her the knife was within the legal limits and she interrupted me and told me that I didn't need a knife at work and to please leave it at home. When I left her office, I promised to obey all laws and to be more discreet in the future, but I didn't promise to leave the knife at home. After all, I told her, which is worse, one 3-inch blade or, and I picked up a pair of scissors, a device with two 7-inch blades? They even came apart in the middle! I kept my promise about being more discreet in the future and did not show my blade outside my office; but I was so pi$$ed about the reprimand (which did not go into my personnel folder), that I carried 5- and 6-inch bladed knives for weeks thereafter. And no one knew that I had them!
Oh, and I never helped those secretaries again.
I also was accosted by two large dogs on the way back from work one day. It was a day that I had left my knife at home. The dogs acted in tandem and one would move on me while the other tried to distract me. Having been badly bitten by a dog years before, I knew these dogs meant business. I had a backpack that wasn't much of a weapon, but I determined which dog was the dominant dog and concentrated on him. I nearly lost a chunk of my leg, but I finally chased both of them off. I called Animal Control and reported it and they were already aware of the situation. The dogs had apparently slipped through a hole they'd dug under the fence and had escaped. After that distasteful experience, I began carrying my 6-inch serrated Cold Steel Vaquero to work. This was in another office, but I still carry at least a 5-inch blade when I'm out and about in the neighborhood. I also got one of those Cold Steel Irish walking sticks and frequently carry that.
Bottom line: You carry according to your needs.