among other things it will eliminate the current problem where you need to use DM if you need some features, but need to use MD if you need some other features. if you need both you are just out of luck
it will also make it much easier to have unified userspace tools. one of the common statements about ZFS is how it makes it so easy to create a filesystem and raid array at the same time. with linux the question would be which raid framework would it support, after this unification it would be able to do everything.
new features (say a checksumming driver) would only need to be implemented once and everything would be able to benifit