[kovidgoyal]

As you say, the fact that calibre created the MOBI file is documented in the creator record. So if some tool needed to differentiate between calibre generated and kindlegen generated files (highly unlikely since there are multiple versions of kindlegen as well as non kindlegen tools amazon uses to produce MOBI files) that tool can check for the creator EXTH record containing calibre. I have just about zero motivation to generate the half a dozen different types of MOBI files and check that everything still works on a Kindle without the 204-207 records. Incidentally, does such a tool actually exist?

--kindlegen is a debug option and as such is not suitable for inclusion in the GUI. I do not intend to maintain it/make it robust. Maintaining one set of mappings from epub to mobiml is quite enough work, thank you. Remember that running kindlegen does not just create the binary wrapper it also re-processes the markup. In most cases this is harmless, but I highly doubt it will be harmless in all cases. And then kindlegen is a moving target, one over which I have no control.