Hey, thanks Dennis! for providing the link to Chris W's explanation. It did answer my questions.
Chris's explanation -- that forcing tag-comments is a means "to encourage collaboration" -- is, I believe, disingenuous.
First, "encourage" is not the same thing as "force". (And Chris, if you are reading this, consider how a government 'encourages' citizens to pay taxes.)
Second, the one-comment-per-tag implementation is, in my opinion, poor. Why was it chosen? Was there any consideration of one-comment-per-group-of-tags? (For instance, "My roots are Germanic, Russian, and Polish. The following tags are for the surnames of my family, with variations. If ever there's a 'one-name study' for any of 'em, COUNT ME IN!!")
However, forced comments is the current law-of-the-land. So I've removed the 18 surname tags for which I am not advertising my willingness to collaborate. The (forced) comments simply take up too much room on the page.
A (negligible) negative result for me: I won't get the automatic emails for activity on those tags. A (potentially large) negative result for the community: someone who wants my collaboration on a surname will have a more difficult effort to find me. What a shame.