I too understand your point Nat. Like Charles a large number of users (65,000+) and adding more for multiple secure sections of a single site. As Charles did, I've created a custom module to essentially filter out those users by what they have registered for. No admin accounts are in any of the searches.
To my point, separating them in the database is not really not necessary since you can restrict access by roles or other properties that Kentico has provided on the user.
Happy coding and good luck with the module, should be pretty easy in 8!