Learning an enemy’s secrets is strategically useful. How useful depends on how one uses it, and how quickly the enemy reacts.

The United States reacted quickly to Manning’s leak, moving compromised Afghan informants away from Taliban strongholds. To the best of my knowledge, no informants were killed as a direct result of the leaks (though it’s possible there are cases that aren’t public knowledge, or where the precipitating cause is ambiguous).

However, assuming none were killed — and setting aside the disruption to their lives to focus on the strategic impact — it still helped the Taliban by removing spies in their midst they hadn’t known about.