Board Thread:General Discussion/@comment-4962906-20151021085039/@comment-25926288-20151022123832

8Rob wrote: Because Dark Magic comes with a price

We saw it in 5x02

No one knows what would be the price for freeing Merlin. It's too risky But all magic comes with a price.

They just don't want the most powerful entity there to use magic, but if Emma did then she could free Merlin and have him help rather than slowly go crazy and get sucked into the Darkness anyway.