In the PvE, this talent works fine with all encounters, currently. Mages receive bonus damage at all specs with 15 points in Arcane.
/Patch 4.0.1 (12-Oct-2010): This talent is now a Tier 2 talent (requires 5 points), down from Tier 4 (required 15 points). Now only affects only Arcane spells. Your Arcane damage spells deal 2/4/6% more damage to snared or slowed targets.
Patch 3.3.3 (2010-03-23): This talent now also applies to Pyroblast damage. Your Frostbolt, Fireball, Frostfire Bolt, Pyroblast, Arcane Missiles, Arcane Blast, and Arcane Barrage spells deal 4/8/12% more damage to snared or slowed targets.
Patch 3.0.8 (2009-01-20): Now works with Arcane Blast and does bonus damage against targets afflicted with any type of slow (such as the combat slow from Thunder Clap).