1. Don’t be too impressed by their theoretical intel-gathering from email– if they were that good, they’d be able to track down spammers.
2. The NSA would probably generate a LOT of goodwill by shutting a few spammers down. Email currently costs an estimated $50 billion a year in lost productivity. That’s a measurable chunk of GDP.
3. Spam is currently estimated to be over 80% of worldwide email traffic. If the NSA shut down half of the spam traffic, the NSA would be able to search what remained much more efficiently.
4. Lord knows, it’d be easy to hide coded messages in spam. Shut ’em down, just to be preventive.