Aegislash is of course a very pokémon used both defensively and offensively. Before making a concrete analysis I want to point out that The fightspam of bw2 is rendered completely useless by this Pokemon, having very high defences and typing steel/ghost can enter on all fight without dark move easily with king shield, now as now without the deoxys, remember to mawile, that would a pokémon from ban instantly, but it is not The time. Aegislash is a great antsipinner, in the stall or in balanced team you can find subtoxic, set that destroys one of his counter, mandibuzz. In the offensives used almost always mixed based on the weaknesses of your team about choosing ironhead/sacred sword, or even used sword dancer, I admit that proves a powerful Pokemon, but not invincible.
King shield just isn't at all how to protect State moves work all right, already the fact of being able to burn for example is a step in countering his physical set, heatran, bisharp, tyranitar, spdefender, Mvenusaur, conkeldurr, mandibuzz, megagyarados, Pokemon are popular in top usage ou and can stop him in almost every occasion, also is a Pokemon very predictable, but nonetheless very strong indeed. With his ban would be definitely the fightspam, destroying the equilibrium of the stall as playstyle and increasing sharply the way offensive, then absolutely not ban!
Think twice before you make snap judgments :)