That's because most gate agents are idiots when it comes to "how it works". Remember I was a gate agent and regular acting supervisor, and I can't even begin to think of how many times a non-rev or jumpseater only got on a flight because I came flying over in a fit of rage to save the day. Nothing about that job, and I mean nothing, bothered me more than seeing a non-rev get screwed because a gate agent didn't know how to deal with a ZED, or an FA jumpseat forum, or transfering a name from a standby list of another flight, or manually doing a CASS entry for a jumpseater but would try and save face by denying them boarding. Luckily, most of my co-workers knew they could just call me anytime they were unsure of anything and I'd take care of it even if I wasn't supervising.
But from what I saw in my exstensive non-reving, I was a rare breed. I once had US Airways station agreement tickets, which are like ZEDs with a $0 value that can be used on any flight, and almost got stranded in PHL with
JordanD because the supervisor didn't know what they were. I had a PHL-SDF segment that was taken by mistake on another flight, I explained that to the gate agent and she told me my DCA-SDF ticket was fine. Next thing you know, they page me to gather my things and get off the flight. The supervisor is standing there with my pass in hand, saying my pass isn't authorized because the city pairs don't match. In the most polite way possible, I ripped him a new one about what a $0 value means, and that even if it WERE a ZED with value, it would be a zone 1 ZED and still interchangable. He knew I was right, was stuttering and embarassed, then asked if I had any other ticket. I showed him all I had was a CLT-SDF pass I didn't use. "Oh! Why didn't you say so?! Have a nice flight...*turns to gate agent* We'll call it a cockpit checks delay".
Know your travel bennies inside out, because the people with the power to get you on usually don't.