Agility is a tremendous asset to any class, but especially one capable as a tank. The premise behind this build is creating an alternative tank, one with high HP/high agility/extremely useful damage reducing/reflecting spells, and AP/MP rape. Also, there's a good variety of weapons with air-stealing on them, great for a tank. Of course this isn't the best build, but it's workable, unique and sounds fun to me, so why not making it?