I would venture to guess that the only good thing about having the "winter" period, would be to jumpstart breeding when the temperatures warm up. Otherwise, I personally believe that it is putting unneeded stress on the fish and will untimately result in a shortened life span. Temperatures that cold will cause stress on the body. Especially for the fancies who aren't bred for cold temps.
I had read in several places that it makes them grow faster during the summer months if they go through a winter inactivity state. I suppose this only applies to common goldfish.