The reason why the precombined Roman numerals exist is that a legacy encoding had dedicated code points for the numbers on a clockface, and one of the goals of Unicode is to provide round-trip compatibility to those legacy encodings. This is also why there is no 13 (XIII) or other higher numbers – they aren’t needed on a clock.
I don’t see a good reason to use duplicate code points, especially when one of them is deprecated; people won’t generally be able to see (and therefore correctly edit) the difference, and sorting is properly handled by ordering relationships. Whether the issue is so important that it needs mentioning in the style guidelines is another question, though.