Character Strings As Primary Keys

Hello,

How bad is normalizing the database to the 3rd form, which requires that all fields depend on nothing but primary key. Consider the first table you create -- users. They have int primary keys, which duplicate the real primary keys -- user names. When user logs in, the user's entry is uniquely identified by its name, which is not primary key. The fundamental design rule -- avoid redundancy -- is violated. A VERY serious reason should be there for that.

Usually, design is compromised by redundancy for performance. Here, both copies are stored in one remote database, but integer keys may be located/used faster. Additionally, using long string references everywhere instead of short integer keys may save a lot of storage space (additionally increasing speed). How serious these impacts are? Am I missing something?

Usually, login names are not allowed to change. You have problems changing primary keys because all the foreign keys must be updated accordingly. Does it reveal that most user databases use character strings as primary keys?