Won't stop monsters either. For that, you need a func_monsterclip with a texture other than clip/origin/translucent. Let's say aaatrigger, that's what I use. Also, to clip a monster, you must tick their monsterclip flag. And with ZHLT, the edge of a sky brush acts as a clip brush automatically, so you wouldn't need 'em for skies.
If you have large sky areas and the player isn't likely to fall through them, you can speed up your compile by using the -noclip command. Clipped skies were introduced around ZHLT 200.(ed:Andy)