User blog comment:DaNASCAT/Technical Update: April 3, 2012/@comment-4785876-20120403172823/@comment-26686429-20120404104431

Ciencia: Whilst unclosed tags are more of a problem on Message Wall than on talk pages, unclosed tags are still a problem on talk pages. They need some logic when a message wall page is saved to make sure that any opened tags are closed (and any closed tabs are properly opened), which should sort the problem.

Your definition of "optional" is "optional for each user of the wiki" whilst Spike's is "optional for each wiki", so you're both correct with your separate definitions. If you really want to use talk pages on a wiki with Message wall, you can use the /talk hack.