This is also one of the top voted feature requests on the UserVoice site for OneDrive - it's sat there for years with no action. I don't get why they wouldn't want to have this, given it would reduce the amount of data they have to store.
probably because they ultimately make their money off people who go over the free storage limit.
storage hardware is incredibly cheap, relatively speaking, so for Dropbox the main thing is not reducing costs, it's convincing people to pay in the first place.
