I understand from posts in the Hunter/Jumper forum, that at some of the big, week-long shows, the Open classes are during the week and the Amateur classes are on the weekends, when the Ammies are off work. So the trainers ride their client’s horses in the Open classes during the week to tune them up, and the Ammies arrive on the weekend to ride horses that have already settled in to the routine. So in that case, there might be scheduling problems if you were no longer an amateur. But this might not apply so much to how Dressage competitions are run.
As far as real quality of the competition, the difference between an Open class and an Ammie class, that would no doubt depend on the discipline, the show, the region. Absolutely I see low-level professionals, riding instructors, who squeak by with much lower scores than competent amateurs, or stick to schooling shows and have no rated score record at all. All “pro” means is that you are being paid for some of your work riding, training, or instructing.
Ironically, IME, pros often end up getting less time in the saddle than a well-funded amateur. They might spend their days teaching lessons, or fixing fences, or cleaning stalls because the barn girl had the flu. They might not be making enough income to afford a nice enough horse to compete at the level they should be advancing to, or have the time to school that horse. They might find that they need to put a priority on taking their junior clients to shows all summer, and don’t have time to campaign their own horse at higher levels. So sometimes even capable, promising pros drop out of showing in order to concentrate on earning an income.
I have to say, I have never heard anyone who had a chance to become a “pro” (even in a very small way) turn it down because they wanted to keep ammie status. They were too excited about the chance of getting paid to teach riding!
This might not however apply to all disciplines, and I think you should look around at your local environment and where you plan to advance. It might matter a lot in some particular circuit. Ask advice of those who know your local situation, like your coach.
On the other hand, I went “pro” in an unrelated field that was my passion and “hobby,” and now do that for a living. It really changed my relationship to that field, for better and for worse. I have absolutely no desire to go pro in horses, because I love having an activity that is entirely free of external constraints and expectations. So for me the big value in being an ammie is that I report only to myself, not to any clients.