Arcane is primarily used by Mages and Druids. It has long been one of the least used resistances, but this seems to have changed with the raid content in The Burning Crusade. Gnomes and Blood elves, having an affinity for arcane energies, benefit from permanent Arcane Resistance as a racial trait. Arcane resistance is not necessary in Karazhan but can be used by an "offtank" on The Curator. It is likely that it will be needed in later TBC raid instances due to the abundance of Arcane resistant gear available.