It might actually be bugged. I have a PlayerModel frame just for some extra effect, and if I assign the points to match the parent on creation it doesn't seem to show the model frame. Assigning the points in the show script of the parent shows the model, but I'm not entirely sure why this is happening.
Lua Code:
Frame:HookScript("OnShow", function(self)
self.Model:ClearAllPoints()
self.Model:SetAllPoints()
end)