One of the problems with saying 'dressage is the basis of all riding' is that it results in inexperienced people trying to incorporate dressage into their riding without having a clue what dressage really is. I'm not a dressage rider, but here is why many dressage books & clinics for non-dressage audiences drive me nuts (and any real dressage riders are welcome to correct me):
Dressage has collected gaits as its goal. In dressage, collection isn't the start point, but the end point of intense training. The goal is a GAIT like what Maura described. And someone willing to devote themselves to dressage will learn that and realize they are working toward collected gaits.
But here is something I've heard 4 times in the last year from local western trainers - and FWIW, I'm a big fan of western riding. Just not modern western riding instructors...
"Your horse isn't soft. You need to collect your horse. Drive him forward with your leg while pulling back on the reins. That will round his back. Get his face to soften...pull it back and collect your horse."
The serious dressage riders on this forum tend to go into convulsions at a statement like that. One of the instructors was one giving me lessons, the others were overheard. And it totally misses the point of dressage, the beauty of dressage and the challenge of dressage. Driving the horse forward with your legs while pulling back on the reins doesn't collect your horse. It confuses him, and pisses him off, but it doesn't collect. If a collected gait was that easy, there would be no need for dressage competitions or training.
Frankly, I think modern western pleasure, like a lot of popular western riding, has been infected with this 'easy dressage' stuff. The roots of western pleasure lie in ranch work, and a guy who rode like this:
For an inexperienced rider on a partially trained horse in rough country, that is actually a pretty good style of riding. And since I've just described me riding my mare in the desert near me, I've tried that style. It is pretty comfortable for both of us.
If you compare that picture with modern WP, it is easy to see the family resemblance. It is also easy to see some changes, and I don't think the changes are an improvement. I think it has adopted some of dressage's style without understanding what that style really involves - hence "easy dressage". The result is a horse that moves with neither the relaxed freedom of a good western horse nor the beauty and grace of a trained dressage horse.
I tend to view the proponents of 'dressage is for everyone' the way I view many popular horse training programs. To make sales, they skip over the real work and substitute easy rules of thumbs - and in the process, ruin more horses than they help.
Good dressage and good western riding are separate disciplines. To understand it, you need to understand the differences between where they came from and the desired end goal. Good dressage and good western riding both start with good riding - a balanced rider on a balanced horse, communicating with and understanding each other. They then work toward their separate goals in a way consistent with their historical roots.
Now that I've stepped on a lot of toes, I'll back away from my soapbox...