With a check box for auto-decline and IF duels were only allowed in instanced areas (like coliseum, good idea) I don't see the harm in it. Duels in open world are annoying to those that don't care for them, myself included - and downright obnoxious in heavily populated areas, cities, crafting guilds, etc.. If the duelers just want to do it "for fun" they don't need an unwilling audience, right?