There is at least one area of wikipedia work that I do which I think HTML comments will come in very handy; HTML tables. HTML tables can get quite complicated and messy, and being able to leave notes for future editors to help them add to the table is something I just don't see the talk: page as being very good for. Very complicated lines of TeX might also benefit from an HMTL comment or two, though I imagine math equations aren't as likely to be modified by future contributors.
In a closely related vein, there are a lot of articles which use standardized "templates"; for example the planetary data sheets or the tatobox for biological species. Filling the template with HTML comments describing what sorts of stuff goes where, and what the preferred formats are, will help to keep these things consistent when new people use the templates.