I have never heard that although I would never use one for several reasons, one being that if a girth strap broke the girth would immediately become dangerously loose so possibly they may not be allowed under some rules.
They are banned in pony club - for precisely Be Positive's reason. If one strap breaks there is no back up. I'm not sure about BS/BE etc. But I would have hoped that people competing at that level would realize they are potentially dangerous.