I've been told many times that it is only safe to back horses out of trailers, no matter what the configuration, but have never been given an explanation for why walking out should be a no-no beyond "it's just safer."
I have a small stock trailer, no divider, and Scout walks out. He does so slowly and carefully, and stops at the door for me to step down before he does. I'm very picky about trailer manners, lol. High on my list of things to teach him is how to back out equally well, just in case he is ever sold and someone down the line tries to back him out or load him into a straight load trailer. I'm a firm believer in preparing horses for any contingency.