Where emergency warrants one-to-many communication + anyone still has a smartphone that can send text messages.
It's becoming common for Twitter to used for updates and coordination in disaster zones. And I guess if I wanted to let my friends know I was safe in such a situation, and didn't want to text them all, Facebook would be a good choice.
Twitter is already optimized for SMS. Why would you access it through this? You just need Twitter to be connected to your carrier.
Yeah that's a bad example, but then there's facebook, email, wikipedia, news etc that can all alleviate emergencies.