Separate accounts for administrators or add a role?

Summary

Trying to determine best practices for administrator accounts

Content

The system I inherited has accounts for administrators that are separate than the person's user account, and I continued that practice as we added admins/super users. 

We are about to implement single sign on, which requires those persons to have two login URLS - one for their personal account associated with their card/SSO, and one for their admin account. 

Since the permissions are assigned by user roles, and we do have some roles that are directly assigned to a user's personal account, I was wondering whether any of you have separate accounts?  I personally don't see any disadvantage to directly assigning, so if there are I don't know of, please speak up.