Why shouldn't I add additional profile fields directly to the django.contrib.auth.models.User table?

To add more information about the user profile in Django, it seems that use AUTH_PROFILE_MODELis the proposed method.

However, are there any drawbacks to adding additional profile fields directly to the table django.contrib.auth.models.User? For me this seems a lot simpler / understandable (although provided, I do not yet understand how to use the signals).

Are there any very strong practical reasons against this?