Add the moment if you enable virtualhosting and create a new user with username "test" it will success. This is bad as for virtualhosting to work correctly you need to have a domainpart in the username like "test@testdomain". So to guard the user from this problems we should make sure we don't allow the creation of such users. The same is true for disabled virtualhosting and having a domainpart in the username. This should not work too..