When to use one field as primary key instead of 2?

Question

I often see some database design like this:

Case 1:

UserTable

--id[auto increase]

--UserName

--Password

--Em

Answer 1

One reason I can think of for not using things like UserName as the primary key is that they could be subject to change. Having anything that's exposed to the outside world as a primary key runs the risk of those things being changed, and it's best to have a stable primary key.

What if the user changes an email or username; do you really want to change your keys in all your relationships? IMO, it's best to have a stable key that never sees the outside world, about which everyone knows nothing, and therefore which can remain stable regardless of what changes may occur in your database.