I believe the advantage is that old drivers still work as they are all in the kernel. With them sharing much code it's not even that big of a disk space issue. Edit: A more dynamic approach would be great though, especially with this size issue popping up.

In a way it's great that I'm able to replace any part of my system and it just works without me having to make sure the old GPU driver doesn't leave some traces behind–altough while writing this the latter part shouldn't be an issue with Windows auto download and installation of drivers.