Roman numerals still presume you're playing on a harmonized scale in a key; not all music fits that as many rock musicians don't know what a key is and pick chords randomly by sound, not by knowing anything about music theory.

I always thought of diatonic as playing within a harmonized major/minor scale, even if you change keys. What I'm talking about is music that doesn't obey the rules at all, doesn't have a key, doesn't change keys, it's just random chords thrown together because the artist thought they sounded good. Changing to another key is still playing in a key. Many rock musicians wouldn't know a key if it bit them in the ass.