We have two trailers. One is a gooseneck 3-horse, with living quarters, fully-enclosed horse trailer. The other is a bumper pull stock trailer, 3-horse, with a large dressing room. On the stock trailer, the dressing room wall swings open, along with all of the dividers, so we can haul hay and things in there (even haul the riding mower in there, to mow the ground of our local saddle club). The entire back swings open, and there is also a sliding door on it (for people to get in and out without having to swing open the whole door). I will never own another straight-load trailer again. They ride soooooo much better in a slant. When you start and stop (or even accelerate a little or slow down a little) their weight is constantly being shifted from front to back. That can (and has been proven) lead to Navicular. When they're riding at an angle, more of their body weight is being distributed side-to-side...and they can even lean on the divider for support.
If in doubt, have someone haul you around in their trailer...driving like they normally do when hauling. Stand in there, without using your hands. Try standing facing the front of the trailer (like a straight-load), then try standing a bit sideways (like a slant). I've done both, and feel it's easier to ride a bit sideways. When I was in 4-H, we had a man drive us all in a stock trailer. Now, he drove a bit harsher than normal, but we all realized just how much our driving can affect the ride.