(This page is still under construction for 3.0. Some of what is here currently is old info)
"Indirect tactics, efficiently applied,
Are inexhaustible as Heaven and Earth,
Unending as the flow of rivers and streams;
Like the sun and moon, they end but to begin anew;
Like the four seasons, they pass away to return once more."
-- Book 5, Verse 6, The Art of War
I've mentioned that I use an Immolation Trap/Serpent Sting/Auto shot combination for farming or levelling with lower alts before, rather than more conventionally using Arcane Shot, but seeing as numbers apparently speak most compellingly with some people, I'm going to try and use some in support of the practice here.
I will use two examples, one at level 26, and one at level 70, in order to demonstrate how the mana efficiency scales.
@ Level 26, Immolation Trap Rank 2 does [RAP * 0.1 + 215] damage for 90 mana.
@ Level 26, Serpent Sting Rank 4 does [RAP * 0.1 + 140] for 80 mana.
We'll assume a base RAP of around 120, as it's very possible to get 100 Agility at 20, if you know about the right gear. That's 367 damage ( +10% of whatever your RAP is) over 15 sec for 170 mana. This means that with 2/2 Clever Traps, you are getting a damage:mana ratio of more than 2:1.
Assuming that as well as using Serpent (80 mana) you fire Arcane twice (2 x 50 mana) in the course of a single mob, you're spending an even 180 mana. (Not an unreasonable assumption; even at level 26, it only does [RAP 0.15 +36] damage)
That means that with Immolation Trap, you're still 10 mana in front of Arcane even without Clever Traps, and with Clever Traps possibly even more since against a higher health mob, you'd possibly need to fire Arcane again for another 50 mana expenditure. As I will demonstrate, this gap scales much higher at 70, and with additional talents.
@ Level 70, Serpent Sting Rank 9 does [RAP * 0.1 + 555] damage, for 250 mana.
Using a conservative example of 1500 RAP, that gives us 555 + 150 = 705 damage. Then if we add Improved Stings, we get another 30% for 211. So our final number for Serpent alone is 916 damage over 15 seconds, at a cost of 250 mana. If you've got 5/5 Efficiency as well, that goes down to 235.
@ Level 70, Immolation Trap Rank 6 does [RAP * 0.1 + 985] Fire damage over 15 sec, at a base cost of 305 mana. Again using our estimate of 1500 RAP, this gives us 985 + 150 = 1,135 base damage. With that, 2/2 Clever Traps gives us 1,135 + 340 = 1,475. With 3/3 Resourcefulness as well, that then becomes 1,475 damage over 15 sec at a cost of 122 mana.
So that means that with a spec like this, you're doing a total of 2,391 damage, at a cost of 357 mana. To put that in perspective, that's less than the cost of four Steady Shots, (about what you'd need to kill a level 70 mob at a 1:1 rotation with Beast Mastery) or two Arcane Shots, and the total damage would be about three times that of either of those seperately.
That also isn't even talking about the threat issue. DoTs do damage per tick on an even basis, whereas crits can cause relative spikes, even at low levels. That means that with Growl being less effective now, it's even more important to cause threat to ramp up gradually, rather than via spikes. If your threat rises via a gentle curve, it's much more possible for your pet to regain threat from a mob even if you manage to pull it yourself.
Obviously this information isn't relevant for raiding or group instances at the same level; but for per-mob farming, it will give you greater mana efficiency and continual uptime than any other spec I know of, including Beast Mastery with a 1:1 rotation.