Hmm yes, that is pretty lacking.
Unfortunately iCloud seems designed with just one person using all the devices on 1 account. And this makes sense for things like document syncing because if you had multiple iCloud accounts on one mac user account you would have to select which iCloud account you want to sync to when using Pages or something like that, which defeats the seamlessness.
For other features like Photostream though they need a separate login I think, so that a family can add multiple accounts to pull photos from. They need this on Apple TV as well...