I feel like two answers there summed it up pretty nicely: 1. Do you want a user's email address to be immutable? 2. Do you want to use a user's email address as

I feel like two answers there summed it up pretty nicely: 1. Do you want a user's email address to be immutable? 2. Do you want to use a user's email address as the foreign key? (Answers: "probably not" and "no, especially if no to 1".)