A 51% attack would be disastrous even if they only double spent one satoshi. It's the loss of confidence in the technology that's the harm. Bitcoin relies on people believing in it to be a secure store of value.
Would 1 satoshi really do it though?
For example, I go to a bar and buy a drink for 1 satoshi (somehow) and it turns out that my satoshi is counterfeit. How would anyone find out it was counterfeit, and once they did do you really think that bar would turn it into national bitcoin news? Think about real counterfeit money, we know it exists, but we don't let it affect the value of our currency, we accept all money as if it's good real money. If it turns out to be counterfeit then one person may be screwed and pissed, but it sure as hell doesn't make the news. Not even the local news, much less the national news. 1 satoshi is probably not a problem.