looks like a caster killer to me. tried the hunter forums on wow yet?

IMExp, I found Serpent Sting is not much useful at high end level. Look at its dmg, about 500+ only about some 10+ secs. I can do that with about 2 normal shots, no mana cost, and 1 Arcane Shoot can give almost that much of dmg. And if I'm lucky got crit, my Arcane hit > 500.
and there's an additional plus on Arcane shot, its bypass\ignore enemy armor class.

I spam Arcane Shot when in need to bring down enemy 1st, else I use it when I got full mana, or as a finishing shot.

You only use Serpent's Sting when you know you can't kill the mob within a Serpent's Sting's duration