It's not just mages that can run and cause damage. Infact, most classes can. In either melee attacks or spells. Also, the TIME and TALENTS you put into your character have a major influence on the damage you can do. A mage decked out in gear specifically for dmg, rather than set bonuses, can get about 1k dmg boost on spells easily. But, mages and warlocks are both frail (Low armor and hitpoints). If you want a pet you can send after someone thats running, or to dmg someone while running, hunter and warlocks are the major competitors, followed by anyone using engineering or trinkets or even items like the dog whistle. In MY oppinion, the hunter class in PvP is godlike when played right, and geared correctly. Many others will argue, but that's their oppinion. Mine is standing by my hunter, who in AV, almost never dies. As long as you know when to fight, when to run, how to run, and where to run, you'll usually live. But, in PvE, the rogue and mage shine as the heavier DPS, followed closely by warlocks, then dps warriors, paladins and shamans, hunters, rounded out by a shadow priest and druid. Though the talents choices rearrange this tree, classes with no talents at 60 with no stat gear or enchants will fall under this tree.
So, play a class you'll like. If you want to focus on JUST spells, go mage. Spells and pets, warlock. Pet and light melee/heavy ranged, hunter. Heavy melee with light armor, rogue. Tank/spellcasting, shaman/paladin, though the paladins armor gives him/her the advantage(Don't start with the pallies can't tank stuff, because they can tank just fine if they try, though no where near as efficient and multiple (3+) mob pulls. Tank/Melee dps/Casting/Healing druid, and Nuke/heal, priest.
Just my fifteen cents
Oh and, locks get free mounts too ;-p