I understand that but I thought when this idea was first presented it was said that the user would keep both key's a in a backup wallet so you couldn't be held hostage with your service provider.
Yes, that's the use-case I care most about -- you have control over all of your keys, you just put them in multiple places so if you lose control of (say) your computer you don't lose your bitcoins.
But there are other use-cases, like you agreeing to let the government control half the keys, so the government can "guarantee" the transactions, etc. I can imagine the PR campaign: "It is just like Federal Deposit Insurance (FDIC), only for Bitcoin!"
I don't think that will ever happen, though. I know I wouldn't trust the government to keep the keys to my money safe and secure, I don't think most people would, either. More likely is most people will trust banks to hold half the keys, and the governments will then regulate the banks like they do today to get information about who is paying who for what....