Plus a hold over the Black Tower. And a chance to hit the Dragon Reborn from a direction that he won't see coming.

Seems like a fair shake to me. And I imagine that the dark one is fully aware of the fact that the seals are breaking on their own(as he is now able to touch the world). What good would holding back this one do for his over all plan? If the seals break in Rand's hands or otherwise, they're still broken.

The Dark has exchanged a worthless trinket for a very valuable commodity, well placed channelers to replace his depleted stocks. Channelers who are in the enemies uniforms, who can inflict a ton of physical and psychological damage as they make their loyalties known.