Even though it has higher defense that Aegislash, Doublade shouldn't be used when you have access to Aegi. Aegi has a few very crucial things that keep it better than Eviolite Doublade. First of all it has King's Shield, which can force many switches on physical attackers when their attack has been dropped and lets you play mind games with your opponent. >:D It also has better offenses in Blade Form and better special defense in its Shield form, which make a big difference.
tl;dr: Don't use Doublade when you have access to Aegi.