Great point. McCain was braced for a war question. There's a reason he got most of the anti-war vote in New Hampshire - he's a veteran, POW, supported the surge that some are equating with improvement in Iraq, and he's great at spinning the "honor" card.
The question Paul asked him would needed to be specfic to avoid letting McCain give a generic response, but the economy is McCain's weakest area, and Paul's strongest, so this was where it needed to be. I only wonder if there was a specific question that Americans would understand and that McCain would still look dumb on.