If you change login and forget to change registration, suddenly "Bob" can't login because "bob" just signed up for the service and your login is now returning 2 users. Django puts username validation on the model field for a reason. It's better to have it all centralized.

The nice part about extending the abstract user from the start is that you can still just use it exactly like you'd use the built in user. There's no difference. It just gives you more options in the future.

It literally only takes 8 lines of code, including imports and a "pass" to do.