The reasons I considered that node for retitling were two: the first being that a person with the same problem as the poster might not search for "trashed by HTML"; the second the title I suggested seem (to me at least) to fit the solution that would and was given.
Considering the amount of questions asked over and over again, I don't think it's that common for people to search whether their question was answered. However, while it's nice to cater for people that come here after they got into trouble, you're forgetting about another class of people: people that visit this site a few times a week to read the threads they find interesting, and learn something. Hopefully to avoid problems in the future. As said, "Disallowing HTML Comments for a Chatroom." is bland and boring, and more people will skip it than if it were titled "Emergency! Our guestbook is getting trashed by HTML!". The first suggests one of the theoretical style meditations you often see here, "if you do X and conditions Y, Z and W are true also, V happens". The original title clearly indicates it's about an actual case. Perhaps the 'Emergency' is overdone. But overall, a much better title.