At one point Arcane Stability was only useful for solo play, especially for lower-level mages that use Arcane Missiles while an enemy is beating on them. While Arcane Missiles can always be recast after it finishes channeling, thus never losing its DPS, the lack of interruption makes it more mana efficient when used while taking damage. For mages that don't take much damage while casting Arcane Missiles, or do not use the spell often, this talent is not as useful.
Recently, especially given the amount of raid-wide damage applied by boss encounters in raid instances such as Ulduar, pushback has become a considerable factor in Mage DPS, dramatically increasing Arcane Stability's relative worth within the Arcane tree. Pushback to a mage's Arcane Blast or Arcane Missiles spell can greatly reduce their effective damage and therefore raid performance.
Since this talent can be combined with the benefit from a paladin's Concentration Aura, which can be Improved, most mages can afford to put only 2-4 points into this ability to achieve close or full pushback immunity.