Why should normal user model never be used with admins?

Would you be so kind to explain why should marking a normal user as admin approach, either using is_admin flag on users table or some kind of roles solution, should never be used? Most php platforms that exist today are using that kind of approach to handling users with special roles.

It is so because of the performance issue if using single model for different ->type<- of users.

what if use have more roles like student, teacher, dean, cafe ....
isn't it cool to have a separate role table instead of creating a guard for many roles