Sure, anyone can walk, trot and canter, but the question is how effective are these gaits when they do them? Anyone can go around the wall in the arena "working" but are they really?
I'll tell you a story -
I board at a very large Hunter/Jumper barn, 40+ boarders and horses here where they are very involved with the LMHJA and blah blah blah.
My dear friend's daughter is going off to college, and they desperately need to find her horse a lease/rider because they cannot continue to pay for his needs while she is gone.
Now, their Daughter has done very well in the Hunter/Jumper circuit compeating at the B level. They always win, and her horse does a great job. He goes in the ring, does his stuff and pins.
So I was put in charge of finding him a leaser. So I've put ad's up and posted him on the internet and have had alot of enquiries. Most are Hunter/Jumpers who come out to try him out.
They tack him up, throw a standing martingale on him, head out to the arena and he does his job. Walk, trot, canter, jump, jump, jump, done. I get alot of "ooo I like him" and blah blah blah.
He goes around, upside down, bracing against the martingale, doesn't track up, back dropped and he says "This is easy peasy, I love my job"
Then, I had a Dressage Queen enquire about him. She comes out, brings her own Dressage Saddle, tacks him up and goes out to the arena.
She rode around on him for about 15 mintues with a loose rein, doing basic stuff, getting to know him and getting a feel for him. Then she asks for a headset - he had no idea what she was asking and it took her about 1/2 hour of repeatedly asking quietly for what she wanted, when he finally got it.
Then she got him tracking up and moving forward and opening him up and using himself. He was pissy as hell. Ears pinned back, tail swishing, not happy. But after about a 1/2 hour of getting him to use himself, his ears were forward, tail stopped swishing, he was lathering at the mouth and happy.
He looked exceptionally stunning with that dressage rider on his back, oh my gosh, who knew he had that in him after all the years of doing Hunter/Jumper.
Sure, anyone can walk, trot, canter, get rhythm and jump a fence - but it takes the strong fundamentals that dressage gives/offers horse/rider teams to beable to do it correctly.
Afterall, I hear so many Upper Level Riders stress time and time again, regardless of what discipline they are - from Reiners, to Hunters, to Jumpers, to Eventers saying how important Dressage is for all, because it helps improve both horse/rider and gives them the important fundamentals to make a more well rounded/balanced mount.
Sure, anyone can go around doing walk, trot, canter, 20 meter circles, and pop a fence - but you can definitely beable to tell which one has dressage training under their belts in compareson to those who do not.
If Grand Prix Jumpers spend up to 5 days a week doing dressage and if Grand Prix Mounts can do minimally level 3 dressage movements, it's because there is a good reason for it - because jumping is dressage with speed bumps.
Yes, if you are walking, you are doing very basic dressage. If you are trotting, yes you are doing very basic dressage. So in sense, yes everyone pretty much is doing dressage whether they know it or not.
But this ties into what Spyder is saying:
So in my reference in saying that discipline switch can be done easily with the horse and rider having basic dressage first simply means that the horse and rider had the fundamentals CORRECTLY trained so both can apply those basics with additional training specific to each discipline and do well.
aka barrel racers, reiners, jumpers, eventers and etc, etc.