Under the assumption that an emergency kit like the ones listed here are for the emergency of getting lost and being stranded for days, how necessary is a knife sharpener?
While it can be useful if you are really out in the wilds, say in the Amazon jungle, and using a machete to clear every step, I wouldn't expect to carry a knife sharpener for a trip under a week.
For short, non-jungle expeditions, A swiss army knife with a couple of blades is often enough for most people.
To summarise - it can be necessary for longer trips, or for overgrown jungle, but for many it won't.
It's not necessary, but something that you probably would want to carry, especially if you plan on using your knife much. Buy a diamond file, very small & lightweight, and you can resharpen your knife with little effort.
I like to use something like this to keep my knives sharp:
I typically just use just one of these, you don't have to carry all three of them.