I guess it’s not totally accurate to say that this is only a historical thing, because Japanese input methods still provide a way to type the fullwidth forms, and people still use them. Most users are aware of the difference and know, for example, that you can’t type the fullwidth numerals into a form field that’s going to try to parse it as an integer.

If you’re setting your text vertically, you would rarely want the regular (halfwidth) Latin characters, because it would mess up what would otherwise be a very regular grid; also they are probably in a different font and there probably aren’t metrics included for laying them out that way. But if it were trailing off at the end of the line, it might look okay… So at least to some people, there is a semantic difference between the halfwidth and fullwidth forms and it’s wrong to automatically map between them.