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
Rank 1 Marksmanship Hunter for Hellfire Citadel, Rank 1 Beast Mastery Hunter for Blackrock Foundry & Siege of Orgrimmar, Rank 2 Survival Hunter for Highmaul.
Icy-Veins Hunter Guidewriter | The quality of my answer will depend on the effort you put into the question.
I have always leaned towards MM I enjoy it the most so thanks for this invaluable info:-)
I start MM and tried the other builds, but always come back
Ya, it's supercool to have a 4.3 MM guide stickied for reference.
Oly damn this is still 4.3 guide, goes to show how many veteran & community active hunters are still around...