If you like Blackburrow and you don't have a specific place you want to go at level 12, then keep them and unload the sauce when you're done.
If you don't enjoy Blackburrow so much and you want to try <zone> at level 12 or so, then dump then along the way so you get there faster and be done with it.
There is no best answer, it's your life, your character, make the decisions that makes it more fun to you
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