For a horse to work in a dressage frame, it takes more work from them than just going as they would do normally. They would much rather just go along very little changed . It is much easier for them to run around strung out and falling forward. It's much easier for me to slump along as I do than to move as a ballarina does. But the ballarina will last longer physically than I will.
Also, one thing I read recently about "collection" said that as a horse comes to be more and more in a balanced and collected way of going, its' four feet will come closer and closer to each other on the ground. The horse will be balancing itself on a smaller area of contact with the ground.
I find that interesting. Something I never thought about.
So, I am going to start looking at photos and see if there's any truth to that.
I've got to disagree with you. A ballerina moves very differently than a fullback because she isn't playing football.
I'm a jogger. Not fast, but I've jogged long enough that I started with sneakers, because there was no such thing as a 'running shoe' back then. I do NOT look like a sprinter, but a sprinter wouldn't look like one while running on the trails I use - too many rocks, cactus pieces on the trail, and an occasional rattlesnake to remind you that you are not alone.
I told the OP that it isn't mainstream to think a dressage position is required, but his response was accurate:
...I guess saying the belief that horses need to be in a dressage frame to be carrying their rider properly is "mainstream," was a bit exaggerated. Perhaps I should have said it was something I have run across frequently. Most notably in people (some trainers) who have done some lower level dressage and probably read some books about the principles of dressage....Some of these people I have run across seem to look at every discipline through the lense of whether the horse is in a dressage frame...
There ARE folks who write books and give lessons saying that a horse isn't balanced unless he's collected per dressage...and that is wrong. But there are books on my shelf that strongly suggest it!
The steeplechase horses in the picture I posted are not strung out and falling forward. They are moving appropriately for what they are being asked to do. The cutting horse in the picture is very balanced and controlled, but he's not doing dressage.
The OP is right - an endurance horse IS balanced and supporting his rider, because you can't carry a rider 100 miles if you are strung out, off balance and out of control.
I don't want my horse to "be balancing itself on a smaller area of contact with the ground." That would be bad on a rocky trail, and not real wise with a green rider who isn't always balanced himself. A ballerina may move that way, but ballerinas don't play football, or jog on desert trails. There are ideas and principles from dressage that I can incorporate into my riding, but my goal is different - so I need to be careful not to cause problems by taking the wrong parts, when applied to how and where I ride.