Toby-
Despite David's arguments, I see one big problem with /requiring/ a <TOC> tag on any article with a TOC. The problem is that articles all start out without TOCs. My solution: Let Erik devise the best system that he can for estimating when an article needs a TOC and when it doesn't. (The current estimate is the number of headers; that's fine.) And let him estimate the best place (before the first header). Then run a conversion script to put "<TOC>" in all of those articles.
This isn't going to happen, but I might implement a <notoc> tag, which would pretty much have the same effect with far less trouble and clutter. For the time being, no convincing arguments have been made for either, though, and not a single real world example has been cited except for the Main Page, which is already auto-exceptioned. So I consider this a largely theoretical discussion.
Regards,
Erik