I’d use that for Advanced Queries and other things that Markdown editors can’t parse anyway, but not for properties because then they would be plain text in other editors, for example:
key:: [An URL](https://example.com)
would be displayed exactly like that i.e. with the Markdown syntax exposed, unclickable links etc.
Since we are at it, if we moved from Markdown to Djot (or at least adopt some of its syntax) we could use Djot’s comments that are like {% comment %} and use the following for Simple Queries:
`Query title`{% query (and [[foo]] [[bar]]) %}
That would be rendered just as Query title in Djot editors:
