There are several cases I ran into where a user did not have an account and it made the entire site not work for them. One case is createuser; I don't remember the other case that happened in production. However, if someone uses a database that does not support transactions or mishandles them, a user could be created and not an account because of an error.

The problem is that if a user ever has the unfortunate trouble of not having an account absolutely nothing will work for them. A terrible experience.