My comment about mages being caster tanks was sarcasm, but...
Think of Magic Absorption + Mage Armor as your primary mana regen. You add Mana Shield and Incanter's Absorption to increase damage, which would increase threat. With other Arcane talents, you can build an Arcane mage meant to be a caster tank given you can find the gear to give those serious boosts to spell resistances and complementing buffs or auras.
The trick for aggro management for a mage is to be the first one to do damage. After that, the mage should have Improved Magic Missile. The problem is getting silenced, and I do not know if Firey Determination has been improved yet.
I don't believe that frost, fire, or frostfire builds need mana. With proper replenishment I've never found myself even needing to evocate, even on Sarth +3 Drakes.
However with an arcane talent build this talent is probably manditory.
As a frost and fire spec, you might as well get it, not so much for the mana regen, but for less strain on healers. Although atm CoH and WG make healing a joke, but when its nerfed its going to be hard on healers and making it easier on them with 2 talent points if you are putting in arcane anyway is a good idea imo.
Well, I tried out a magic /incanter's spec tonight and did 10man naxx. We had some new people and had trouble with tanking zelik and blamaux, so i went back there and tanked. Zelik was painful, but blamaux I was resiting 1/3 of the dmg from every shadow bolt.
And there were actually a lot more fights with chances to pop wards and mana shield to absorb some damage and boost my spell power. Sapphiron obviously, and frost ward during KT worked well during the frostbolts that came out occasoinally.
Maexxna I mana shieled and grabbed te spider adds. AoE burned them down and gained a decent spell buff.
Thaddius I popped mana shield when polarity got highly stacked on one side (6-7) and absorbed the chain lightning for some nice buffs.
Grobbulus I popped mana shield when I got injected and came back to the fight with some extra spell dmg.
During sapphiron it helped with survival a lot. With a short evocate cooldown, I used frost ward whenever it was up, and if blizzards got bad I would also pop mana shield (since mana shield scales with spell power, i got extra protection for keeping frost ward up before hand).
At any rate, I was quite satsified with speccing for absorption for raiding; I ran some BGs earlier in the day and it was a great boost there as well.
However, I did give up Icy Veins for the spec, so I'll have to try out a spec this weekend using IV instead of absoprtion and see how things are.