License borrowing with Maya does not license mayatomr plugin
When you try to use mental ray for Maya with a borrowed license, you will get an error:
(Mayatomr) : could not get a license mental ray.
When you borrow the license, you will need to perform a Render > Render Current View to use the mayatomr plug-in before exiting Maya.
In the documentation under license License Borrowing:
(Optional) If you plan to use any sub-features of Maya (such as fluid simulation, mental ray rendering, or the mental ray local batch render) while using a borrowed license, use those sub-features now while you are still connected to your license server. This automatically borrows a license for those sub-features.