I don't see how kunrei-shiki is useful at all. If I want to write Japanese words so non-Japanese speakers can pronounce them approximately, then Hepburn is the way to go. If I want to write Japanese words so Japanese speakers can read them best, I'll write them in actual Japanese. This isn't 1975, and computers are perfectly able to render hiragana, katakana, and kanji. What do I need kunrei-shiki for? I've been living in Japan for years now, and have never found a use for it.