I agree that this would be a good feature to have, although the suggestion that it should be easy because the Importer system already has a merge user system is invalid, as the importer does not have to work with existing data, it only works with data that is to be imported.
In order to work with existing data, we must take into account data belonging to users that is not part of the XenForo core. To that end, a solution must be developed whereby add-on developers may register the content types of their applications so that they too may have their authors merged.
This will require consultation with add-on authors in order to agree on the best approach. I have a few ideas that should work, but I will need add-on developers to sign up to the scheme in order to make it fly. Perhaps we could organise an online chat sometime, if Floris would agree to host it in his IRC room.