Seriously, how expensive are hard drives these days?!
But I think that's mostly just overkill. I'm fairly sure that it is completely infeasible to recover data even for the mighty government. :P
I suspect the theory is that wiping a drive is prone to accidental failure/mistakes. Destruction is pretty unequivocally permanent :)
After all, simply destroying a drive would allow you to magnetically scan the bits and pieces of the platters left over.
That would be an interesting challenge, now to find someone to pay for it.
I don't think it would be easy, but if the information is still on the platters you should be able to recover at least some of it.
Especially if it is email or other textual information it would not take a very large fragment to contain a large chunk of text.
On a single platter drive you'd have a better chance than on a multi-platter one, the synchronization tracks would help in figuring out what went were. It's a bit like puzzling together shredded documents.
The other problem with this is that the NSA is paranoid, and paranoia doesn't necessarily move with the times. This relates back to the first thing: If it's cheap and safe to destroy drives, and you are storing secrets, why not? That does not mean you need to destroy drives, which is what this conversation is about.
Wearing a belt and suspenders does not mean belts are prone to failure.