_distancecull just cuts off visibility at the given limit. ie, the engine stops drawing. So anything beyond that number isn't rendered, and when the engine doesn't render stuff, you get HOM.
_farplanedist is used in conjunction with _foghull <path to sky shader>. It does essentially the same thing, stop drawing at the given distance, but instead of not drawing ANYTHING at that limit, the game draws the specified skybox. Check here (
http://www.shaderlab.com/q3map2/manual/foghull_guide.htm) for more explicit instructoins on how to use these keys with a foghull.
You can use a foghull skybox without actually putting in a fog brush or a global fog, but it might look wierd...remember, the game will stop drawing things at the specified distance! It's best to have objects FADE out into the distance instead of just disappearing suddenly. You could disguise such an effect by having a canyon-like terrain instead, with bends and twists positioned such that any given open space is always smaller than the _farplanedist. But if you want a FLAT open terrain, fog is really the best way to go to use a distance-culling technique. You might be able to get along without it...try it and see.