Honestly, I'm not entirely sure that was the originial intention of the GAC system. Now, the start of the GAC program was before my time, so I don't have first-hand knowledge of its origins, but this is how it has been explained to me by a handful of seasoned cast and leaders:
I'm sure GAC was not intended to help non-transfer wheelie folks like me. That's why I never thought to ask for a card the first couple years it existed. It was supposed to be for guests with invisible disabilities. Not something a CM could detect at the gate.
Back then guests needing that special wheelchair accommodation were directed to special wheelchair queues. IASW had you report to a CM and then go to the Exit and proceed down the ramp to wait for service. ECVs were not even allowed inside. Sometimes you'd get wheelies transferring but usually we all just waited for the wheelchair boat to cycle through the ride. Then they went to FPs and mainstream access for the ECVs. Now you could take an ECV into a queue, but those who needed the special cars lost direct access to that queue.
So CMs started recommending those guests get the GAC allowing them to bypass one queue to get to theirs. But it got confusing, because some GS CMs gave you a GAC with a wheelchair stamp. That was redundant because it was meant for guests who used strollers as wheelchairs or ECVs as wheelchairs. If you were in an actual wheelchair that was pretty obvious.
It doesn't surprise me EPCOT was not in the habit of giving out the "Alternate Entrances" stamp. Most of the attractions there are either movie seating with wheelchair access in the back rows or involve very inaccessible vehicles that require a hard transfer (e.g. Test Track where you do a difficult lift and slide transfer down into the vehicle). The two that have wheelchair cars tend not to be busy or have separate lanes for wheelies inside.
Spaceship: Earth is a unique exception. It operates the same way it has since 1998 for wheelies. You go inside the exit, report to a CM and wait for boarding. CM takes your name, number of your party and gives you an estimate for a wait time. They try to factor in Standby times outside to be fair. I always go when the crowd has moved on to the Countries for the day so I'm just waiting for that new slide-transfer vehicle to cycle through the ride. (I wasn't able to ride this attraction before they had that. It was like Haunted Mansion and Snow White for me. One I could only do if I had helpers to lift me. Now I can do HM mostly on my own. Snow White never got accommodation before it was taken down.)
Ideally I'd like to see more attractions do that or what they do at Star Tours. There you are sent up a separate wheelchair ramp to wait for boarding. Gets you away from the pedestrian throng and let's them process wheelie guests much more efficiently. Used to be you boarded from the front. Now you are escorted down a hallway to the exit and are assigned a vehicle to wait for. When that unloads, you can drive your chair down the ramp to the vehicle, do a slide transfer and then ride. You exit the same, except someone brings you back your wheelchair. CMs just have to remember to save some seats for your party because while you are loading via the exit, the Standby folks are loading from the entrance.
I plan to have talks with GS next week and explain my situation. I'll also try those rides I now do with the GAC and see if they have some different instructions for guests such as me. I can see the abuse if they just willy nilly hand out return tickets for everyone with a wheelchair. It shouldn't be for guests in ECVs, or those with little to no transfer issues. I'd be happy if I had a stamp, card or even a wristband that says I can only use DAS if I ride in my wheelchair. Most abusers will not want to do that.
FWIW, CMs have always tried to be very helpful to me when they can with just a few exceptions. CMs directed me to get the GAC so they could help me and still follow their rules.