Actually, several purebred Friesians have competed successfully at Grand Prix level dressage. At least two of them, including the late Goffert, are from Iron Spring Farm. I receive this farm's yearly DVD, and it always includes clips of their Friesians in competition.
Ritske R, a Friesian stallion from Ots Sunrise Farm (very near where I live), has competed at 2nd level with scores in the mid-70s and is training 3rd level. He has been a USDF Region 4 reserve champion. Dressage judges used to be predjudiced against the breed, but that is changing because the Friesians have really been proving themselves
While I wouldn't recommend a Friesian for jumping, some of them are very athletic and quite suitable for upper-level dressage.