Some people could claim that calibre's purpose is to be whatever the author intends it to be, in this case, a one-stop shop for anything ebooks. That includes editing apparently, which isn't really all that different from conversion.
dual-MOBI is not very often used, and it is sufficiently obscure that it might have flaws in calibre's handling of it.
Used trumps original scope in my books any day.
That being said, I would love for calibre to handle dual-MOBI better, if just on principle. But I don't blame Kovid for not doing so, since he has mentioned it before and said it would require an inconvenient amount of testing to make sure existing, widely-used functionality didn't break, in order to fix something that few use.
Me, I'm not much of a one for principles, I prefer plain old practicality and focus on priorities -- I suspect ultimately spellcheck makes more people happy than dual-MOBI handling updated metadata.
