As far as I can see, the sites' FAQs and this Meta site do not address the purpose of comments. What is their raison d'être? Are they meant to be permanent or ephemeral? What should they contain? Until we sort those more fundamental questions out, I don't think we can really discuss whether they should allow for blockquotes, or LaTeX, or what-have-you. So have a downvote from me not because I disagree with your proposal fundamentally but because I disagree with it right now.

@msh210 comments are meant to be, well, comments. Constructive comments to improve a question/answer, or like you just did, comments to state why you disagree with something.

Dang, people really don't want to have blockquotes in comments for some reason...

I've used italics for this purpose. Seems to work well enough and doesn't eat up a lot of space or add visual clutter like a blockquote would.

@Olin Lathrop ah, that does seem like a good workaround. I'll probably use that for now.