My personal feeling is that we shouldn't use them in our conffiles,
Agree. I think that apart from the reasons mentioned above, it just looks nicer in the conf file when all commented-out lines start with `#`.
but that we should support them in the file type; I don't see any reason why to limit this, is there really valid cases where it causes problem to support it?
I don't have a strong opinion on this. There are two possible views:
1. Display all possible conf files nicely syntax-highlighted. 2. Encourage users to use the safest conf file format possible by not highlighting a possibly invalid format (the specification here https://specifications.freedesktop.org/desktop-entry-spec/latest/ar01s03.htm... doesn't say anything about leading or trailing spaces so it may be implementation specific).