We called it club courtesy. That and industry nights spoiled me forever. Think the last concert I paid to go to was Massive Attack in like 2005... which was also the last concert I went to.
From years of doing a job that requires going to concerts: it's so easy to see free shows it would blow your mind. Unless you're looking to get into an arena-type show where the security is out in force, it's entirely a matter of just looking like you know what you're doing. Nobody ever checks badges if you just pick up a road case and walk backstage with purpose. Ditto shining a flashlight on the ground with an intent look on your face. Or anything with gaffer's tape. Who needs job leverage when you've got good bullshitting on your side? I highly recommend that everybody try this at least once.