Nobody does what you describe. When someone has a genuine need for local admin access, they are given two accounts - an unprivileged account to log in with, and a privileged account to enter into UAC when needed. It would indeed be an unbelievable waste of time to make your IT staff sit next to users to enter admin credentials, which is why nobody does it.
And for the record, "don't log in with a local admin account" is a very commonly recommended best practice for Windows environments. It's unusual that you've never encountered it.