There is still a difference with regards to a hard summer pitch and a hard winter pitch though - in the summer the hardness is down to a lack of water and the ground is usually dusty and still allows a bit of 'break-up' and a bit of give. Whereas in the winter a hard pitch is 'harder' IMO, aswell as being a tad slippy due to the ice.
As a keeper, a summer hardness is also a little more forgiving on the body than a frozen hard pitch. No idea what the scientific reasons for this are but thats my general opinion of it.
I probably make no sense whatsoever with the above but it seems feasible in my head atleast