In reality, Stienberg (IMO of course) should have never setup the file system like they did. A .cpr should just be a .cpr and open in any version of cubase.
Pro tools files open in any version of pro tools, you can open a pro tools LE project in HD and then re open it after working and saving stuff in HD back up in LE again.
There really isn't any good reason not to be backward compatible.