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.
If you are only carrying a knife for emergencies it is unlikely that you are going to need to sharpen it over the course of a few days, assuming of course that it is properly sharp to begin with.
However as studiohack rightly points out a diamond file is so lightweight and a sharp knife is so important that it does make a certain amount of sense and you can buy unbranded ones very cheaply.
If you are going on any kind of expedition where a knife is an everyday tool then it is a different story and a basic sharpening kit is essential.
All this of course assumes that you know how to sharpen knife effectively.