So you trade off a ton of single-target DpS to pad your meters where it totally doesn't matter and, in doing so, put yourself in melee range so you have a MUCH higher chance of dying and slowing the group down completely? Trash just doesn't matter. You aren't doing anything but stroking your e-peen by speccing for AoE. A 1 second difference in trash packs' ToD isn't going to do much to change a run. At best, that's like 40 seconds saved (assuming 20 mobs and 2 seconds saved, which you will not be doing as Arcane is only a small drop in AoE DpS).
Now, bosses on the other hand, have much higher health pools. Doing 2K (or in your case, 4K) more DpS is a huge advantage. Assuming your group averages 8K DpS now (2K from all DpS and tank), bosses like Krystallus will go down in 54 seconds. If you did 3K more DpS, he would go down in 39 seconds, a 15 second difference. Just 3 bosses and you've reduced the run time by more than speccing for AoE did. And every instance requires you to do at least 3. And MANY have far fewer than 20 trash packs.
Furthermore, in instances where trash mobs would live long enough for a LB spec to be worthwhile, which is basically the Frozen Halls instances, I would STLL say a ST spec was better. In FoS, I would much prefer a Mage be able to pump out an extra 3K DpS, to ensure that those spheres go down before they heal the boss--kiting him's a *****.
In PoS, there are very few trash packs that are 3 or more. And, of them, it is generally best to focus fire as 2 (well, 3) of the mobs in the packs put out a LOT of damage and healers and tanks need to work hard not to die from them. Reducing their damage is more important than killing the whole group faster. Furthermore, the higher casting time of your signature nuke compared to AB makes fighting during the Arcane Bomb portion of Ick much harder. On my Arcane Mage, I can just fit in one and run before getting hit. Best fire could do is keep up LB.
HoR would be the only instance where an AoE spec could possibly be okay. However, tanks are usually fighting DpS on threat in that instance--you should not be placing yourself in melee range. On top of this, the mobs have AoE, so it is wisest to be as far away as possible to do your best to ensure that your healer reaches the boss with mana. However, HoR is still not that conducive to an AoE spec except in the most geared groups. Anyone before that usually performs better with targeted killing, so as to take out the priest and high-damage mobs first. LB would be fine, but I would advise against anything more than that if your tank is marking.
Naturally, in ToC, an AoE spec is a joke.
You should be clearing everything else so fast that you will not see a single LB explosion there. (2K DpS average for 4 players is enough to kill all those mobs in under 12 seconds).
And coming from a tank, I would much rather DpS spec to do single target DpS than AoE. Mages especially are on of the classes it is hardest to hold hate on (even Arcane ones who do sightly less DpS than you). They are easily my highest casualty rate. And if they all stood in melee range, that number would be way higher (if you didn't know, when outside of melee range you can put out an additional 30% threat before you pull the mob, melee only 10%).
This may be a different matter if there were significant adds in boss fights in Heroics, but the only ones there are generally only produce 1 or 2 targets (or produce them in a huge area, so Blizzard wins by default). Though a fire spec would be useful if you are still gearing (so like, 2pc T9 or less) in OK. I can kill those guys in 1 or 2 FSs, depending on crits, now. But it took way more work early on.