I've not shown, but I ride a Friesian mare in my dressage lessons. I've seen my instructor REALLY ride that mare and she can move.
Of course there will always be horses better built or better "suited". However I just don't see that being the main point. I'm cheesy and so unbelievably far from being an expert at anything horse-related but I, personally, see the main point as enjoying the sport and improving one's riding and the horse's way of going.
Some people like dressage. Some people like Friesians. Some people like both! I think if someone wants to ride a Friesian in dressage then more power to them.