A household has many people that can use the Echo. Having a userid per household is not that useful. It would be great to have a user management component that tagged each utterance with the identify of the user based on their voice print.
I believe this is done with Profiles already. For example, my SO's voice isn't recognized when Echo is in my profile. She has to say "Alexa, switch profiles" and have Echo to go to her profile before Echo really understands what she's saying.
It's good profiles are there, but the overhead of switching profiles makes the experience awkward and unnatural. Alexa can just know. Also for security issues like preventing purchases from children, for example, the manual switching doesn't work.