Clannfear is an excellent tank although a lil weak but is fantastic for keeping aggro.
There's your answer. When you are in a 4-man or 12-man group who has a proper tank to keep aggo, and you're a DD, you'll want to dish out as much damage as possible.
Clannfear good for solo because it keeps pressure off you. But for sheer damage output the volatile familiar is better.