Brogan originally brought the idea to attention but it does seem as if everyone is still posting about admins logging in as users
Any who... other software already have this ability and I love it, I've used it so many times.
Basically the idea behind it is an admin can click on a user-group in the admin panel to obtain the same permissions as that group. This would allow the admin to quickly browse over permissions of different groups in a live mode rather than viewing tabled information that could be overlooked potentially. Somewhere on the page as you're browsing there's a simple "Revert permissions" or "Back to admin" which would restore your permissions.
// ACTUAL IDEA
Some say this should be a hack or an add-on but I would rather trust the developers to implement this haha .
Some will also say - just make an account to test out the privileges - well so many features that are built into software are put there for EASE.
I've attached some screenshots in my post here