Ricardian, while the US system (also applies to NATO) has some benefits and it was applied in UK for a while, it has disadvantages too. First, it highlights to someone without the need-to-know what is most sensitive, and secondly it relies upon the originator to identify accurately if a part of the whole can be safely accorded a lower classification.
I recall once being 'told' the content of a Secret caveated letter from a 2*. Later the letter was reassessed, and the inclusion of just one word meant the letter was upgraded as a whole to TS. As TS I was not only allowed to read it but was responsible for filing it too.