Because the pin would sit horizontal, it is no longer secured by gravity it could rattle loose. I've carried numerous loads in the back of my jeep over 500 lbs. If I switch to a drop down tail gate, those loads would need to rest on the tailgate while being loaded. I doubt I'm the only one who sometimes uses their jeep as a small truck.
Once you put your idea on a forum, its no longer "just a jeep tailgate." Before I knew better, I blindly followed recommendations of others online only to wind up worse off. People often show up with excellent ideas that are half cocked or prone to eventual failure and fail to note their downfalls. My only interest is to make the best project I can, and am only looking out for others who are interested in the same.
Anyhow, if you use the door hinges, get back to everyone in a year or so, if be genuinely curious how it works out.