I do it out of necessity, but it is horrible. And embarrassing, considering my father was/is a horn player.
Anyway, I write mostly for natural horn as I prefer its sound to that of modern horns. I also score for the horn as though I am an 18th century composer. If the work is in A Major, than the Horns are in A. If the key is E-flat Major, the horns are in E-flat. If G, than they are in G, and so on and so forth. (As though they use crooks.)
However, there are a plethora of issues that arise:
What notes specifically can it play- what is the harmonic series? That is, what pitches and/or scale degrees are actually playable on the horn of Mozart's and Beethoven's day?
Also, what is the range- how low can it go. I've seen bass clef notation in Beethoven (the first bars of the 7th symphony for example) and Wagner (to my knowledge he wrote for natural horns- most scores of his operas give horns more keys other than "F")
This is driving me insane and has really sapped my desire to write lately as I'm not entirely sure I like what I have for horns in some of my works. I suppose I could scrap it and just boot the horns out of my works (which I'm not OK with, but I'm even less OK with not writing for a few days). I want to go into filling in their parts with clear knowledge of what they can and cannot do.
Disclaimer: I REFUSE to write for valved horns unless the work itself requires it in my estimation. But I will always prefer the sound of the natural horn, particularly in works where the winds and strings carry the majority of the work. What can I say, I am not a huge fan of the brass section.