I don't think it's necessary to work with an international or national trainer, to get some help for the QH ring.
They are few and far between, anyway. There are not very many of them. And they tend to be expensive and hard to get a hold of because they are often traveling or are very, very booked up. If you were to want to focus only on dressage and want to go up all the levels, then a person who has trained a lot of his own horses (and students) up the levels would be indispensable.
That said...I can see why anabel would recommend a more experience,d, higher level dressage instructor. The experienced instructor/trainer has a far, far better grasp of the whole training process. The less experienced instructor quite often, not always, but quite often, tends to get caught up in details and not see the whole picture.
In a perfect world, we'd all be learning from the best in the business, of course. We'd roll out of bed and go down to our barn and meet the world champion trainer in our chosen sport and learn from the best.
There are a lot of 'grab bag of tricks' type dressage instructors at lower levels, that do not grasp dressage well enough to be creative, and see how it could be applied to a Western rider. Others aren't flexible enough to adapt their dressage instruction to what the QH classes want. The more experienced an instructor, the more s/he can adapt things and be creative.
And...there are dressage instructors who actually specialize in work for the western classes, or work for jumpers, or for eventers, and have a lot of experience doing that. Someone who used to be an eventer, or a show jumper or a western rider, before dressage, can have a special understanding of the jargon and methods and be able to communicate very easily, what to do.