A MusicXML export integrated into LilyPond has been discussed repeatedly, and everybody agrees that it is desirable, but it has not been implemented so far because, well, it's quite easier said than done. In a free (libre) / open source project, the only way to be sure something will get done is to do it yourself.I didn't want to sound like complaining! I understand it should be a lot of work to provide. I wasn't sure it was not implemented for technical or ethical reasons.
I understand. I would say it is unimplemented not for ethical reasons,
nor even for technical reasons, but simply resource reasons — unlike most
of the other problems that are discussed frequently (grace synchronization,
cross-voice spanners), it isn't made hard by the way LilyPond is designed,
it's just a big chunk of work that needs people with all of time,
motivation and competence to tackle it.
