I am also confused as to why one would need untranslated subs in undubbed content. Maybe we are confused about whether we are talking dubs/subs?
It means why write "Dunamisdeos-san" in the subs when you could write "Mr Dunamisdeos".
However, the counter-argument is that honorifics are culturally specific, and translating culturally specific things into what are only very loose approximations always messes things up. Like, you wouldn't translate "ninja" to "spy", even though technically, ninja is untranslated. We could just write "futon" as "mattress" too, and "samurai" as "warrior" or "swordsman". So, as you can see, dictating that everything
must be translated to the nearest equivalent is equally fucking stupid.
So, the point
has to be that we leave things untranslated when the non-translated version is more accurate and commonly known, for a very subjective line of what "commonly known" is. For example, onigiri become "rice balls" (or "jelly donuts" if you prefer) because the name onigiri is less commonly known in the English speaking world, but sushi doesn't become "fish rolls" because the name sushi is already commonly known. So, for our "full translation dammit!" demand we get upset if they
don't translate onigiri to "rice balls" but equally we would get upset if they
do translate sushi to "fish rolls". So, no, there is no actual set of rules we can apply here, it's always subjective.
Honorifics are therefore middle-ground, because they're culturally specific, and that's where those works are set, and it doesn't take a rocket scientist to pick them up.