Understanding Database Design

Hey Guys,

I am working on a plugin for importing users into wordpress. Since I need more information about the users than the table users provides to store, I store additional information in the Table usermeta. Here the question came to my mind, why the architects of that database did not choose the pair (user_id, meta_key) to be unique? In that case, I could easily update the table with the keyword REPLACE without having dublicated entries.

BlueShark