I have a very different view to @mwra’s. I think it is a fine use case to view rendered text in Preview and never export it. This happens to be my use case, FWIW. And, backstage, I lobbied for this feature, so I’d like to think that helped influence why $HTMLPreviewCommand came about as a Preview but not an export feature.
The attribute affects nothing about exporting. FWIW, if one wants to export markdown text, it is best to export the plain text and then use Marked or other software to render it. (I’ve posted tutorials on the old forum explaining that.) Marked is more feature rich for markdown rendering than Tinderbox can be without considerable heavy lifting.
(There are other ways to get markdown exported, but that’s not the topic of this thread.)
@jmm’s point about map’s not displaying rendered text is a good one – and I think the limitation is that Maps are not previews, and making them act that way is perhaps a bridge too far.