[Steven D'Aprano email@example.com]
I don't know what "multi quote" means, unless it means quoting multiple people's text in your reply. (Which I can do in email by copying and pasting.)
Can you link to an example of this useful multi quoting please?
Sure - here's a message in which I included bits of three other peoples' posts:
This is "better" than email copy-paste in several ways:
to quote, and a "Quote" control pops up. Click that and the selected text is properly formatted in your reply, automagically attributed to the original author, and automagically linked back to the original post.
quoted text. Clicking one jumps directly to the message you quoted. Clicking the other expands the full text of the quoted message inline, with the part you quoted color-highlighted.
That last is my favorite. It's great to see whether important context was snipped. And, once you're used to it, I expect you _will_ snip "important context", because it's so easy for the message reader to just click to see the full original context - if they want to.
So, in the example I linked to, the quotes I included were very brief. In email I would have quoted much more, because it's so hard (at least in Gmail) to _find_ the original messages again.
The "multi" is a nice thing, but all of the above applies too when just quoting from a single source.
In the context of the message thread you were replying to (which I'm not quoting here at all, because it's such a PITA to find the original bits in Gmail), the "theoretical point" was that multi-quoting doesn't play well with a threaded view: your reply is "a child" of _every_ message you included pieces of. The "message tree" is no longer a tree. Which I really don't care about ;-)