Originally Posted by
Restrung
With your Aimed shot proc, Master Marksman, and the amount of times it will proc in a fight, Arcane shot will do more damage over time than casting a full timer aimed shot. That being said, opening with rapid fire and a potion you should be using aimed shot as much as focus will allow until rapid fire is up, also whenever you have rapid fire available.
Haste heavy MM hunters have the advantage of using aimed shot with Starcatcher Compass, if you have an addon that lets you know when it procs. I use the heroic version and the haste increase puts me well up in the mid 3000's for haste. Its nice because your aimed shot cast time is reduced to almost 1 second...but I digress...
For a hunter in regular 378 gear or even 384, the mm rotation without any haste buffs should include arcane shot. The loss from a 2 second cast is not worth it. And because gear has gotten better from 4.1, the damage of arcane shot, along with the 5% increase in damage from the glyph, you'd be stupid not to use it