Join Date: Jan 2009
Location: USDF Region 1, USEA Area 2, USEF Zone 3 - Maryland
In eventing dressage you can totally use a breastplate.
I'm sure you can at smaller schooling shows also, if you need to. Most people just don't use them because of their potential to limit shoulder freedom. They just keep on saddle shopping until they find one that doesn't shift. (At least this is what I have seen around here).
The only areas of competition where I would be skeptical would be the FEI levels.
-Melanie Mom to 3 bays: Beau, Daisy & Cavalina