I think it’s the wrong call to categorize this as a “feature.”
I could imagine saying “this bug is too hard to fix; it doesn’t make sense to prioritize it over other work.”
But I don’t think anybody would defend the current behavior as correct.
As for your question, it’s not really possible to give a “use case” for a bug fix. Features have use cases (Blur Spoiler: Users want to blur spoilers so they can discuss media without ruining the surprise), but bugs reside inside features. Fixing the bugs is how a feature fulfills its use case.
Why does this bug matter? Because we use spoilers a lot!
If I treat this issue as a “bug,” and acknowledge that implementing my proposed solution might be expensive, the closest I can get to answering your “use case” question is to answer a different question:
“Why does this bug matter? Granting that the current behavior is incorrect, who cares that you can’t blur inline text across multiple paragraphs? Do you even really need to do that?”
And to that, I think I’d say: the current experience is just confusing, and undermines the user’s trust in Discourse. When you select some text and click Blur Spoiler and it simply doesn’t blur the text you selected, it’s just embarrassing for everyone involved.
Honestly, it would be a mild improvement over the current behavior to display an error message when a use tries to spoilerize parts of two paragraphs, educating the user about the nature of the problem. The error message could say: “In Discourse, you can spoilerize part of one paragraph, or you can spoilerize one or more entire paragraphs, but you can’t make a spoiler containing parts of two or more paragraphs.”
But now imagine if you had to show an error like that for bold text? Or italic?
And that gets to why spoilers matter to me: the forum I operate (and other Discourse forums I participate in) are gamer forums, where talking about media, and especially without spoiling the solutions to puzzles, is a really big deal.
I can understand why someone would say “Spoiler blur isn’t as important as bold text. We’ll fix the bug for bold by making multiple bold sections, but for spoiler blur, we’ve just got bigger fish to fry; let’s leave the spoiler bug unfixed. We just don’t care about spoilers that much. Users will find a workaround.”
But for me and my forum, and the forums I live on, spoiler blur is marginally more important than bold text. That’s why I’ve been pushing on these spoiler blur bugs!
What’s the “use case?” The use case is: we use spoilers to talk about media without ruining surprise. And so, the blur spoiler feature should work, and work correctly, to satisfy that need.