I think flapping is an amazingly cool communication! Last year, when we had a less than thrilling OT eval by the school system (where I had to explain stimming to an OT) the teachers and I had a great conversation about how you could read my Aspies internal state by just observing his flapping/drumming. He's got happy/excited stims and frustrated/about to melt down stims.
Julian eventually switched from flapping to drumming on his body, but it's just as readable and an invaluable tool for someone observant enough to understand this visual language. This is the ultimate expressive receptive and emotive language, even if it's not what speech therapists evaluate for.
I say flap on.