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.

> Also I have never heard anyone suggest you shouldn't use a local admin account.

Only every security audit under the sun. If you work for a sufficiently large organization subject to industrial or governmental regulation, or even carry particular insurance policies, third party audits will flag these practices as liabilities, because its boilerplate recommendation for how a managed windows environment is deployed.

You may work for a large organization where you get local windows admin. Exceptions can be made if a good story can be told about compensating systemic and detective controls that sufficiently mitigate the risk.

However I promise you that someone in that organization closer to security strategy and compliance gets grief over the posture at least annually. Those people shelter you from worrying more about it.