Right now at level 80 (i dont know how it will be at level 81+) serpent sting just feels like a waste of focus for beastmastery hunters and i dont use it at all in my rotation because... it costs just as much focus as arcane shot but deals less damage (i have glyph for arcane shot) so why would you use serpent sting over arcane shot at this point?
Cobra shot might make it usefull when cataclysm hits but im not sure