It's useful to keep in mind there's two layers to the Snowden-betrayal array of claims.
- There's the claims that he did damage.
- there's the underlaying claim that this proves that he did wrong.
In fact whenever a whistleblower comes out, there will be some damage in some areas. The same applies to journalism. Whenever you expose wrongdoings or questionable practices from those in charge it can be argued this helps the enemy, even if only by tarring the image of the government. But I think the main point is, it should be considered an acceptable cost of transparency of governance. Transparency has been embedded in the US constitution 200 years ago for a reason. Mostly, those accusing Snowden don't understand that reason, or see no reason to bother with it. Transparency means that to some extent the governing still represent the governed(although you need to close the feedbackloop to really achieve that).
So yes, I think the claims that Snowden damaged the US foreign policy are wildly out of proportion, but I also think that as long as some precautions were taken to limit damage done, then it's acceptable. That should be the general attitude towards whistleblowers: that some damage due to disclosures is acceptable, worth it.