To quote from the phpdoc:
* @param string $value Thingy goes into a wikitext table; it used to be escaped but * that was incompatible with previous practise of customized display * with wikitext formatting via messages such as 'exif-model-value'. * So the escaping is taken back out, but generally this seems a confusing * interface.
It appears that in general $value is expected to be wikitext, but in htmlspecialchars is usually used to escape raw data from EXIF tags, etc, instead of wfEscapeWikiText so that URL auto-linking occurs, and parameters to messages are not necessarily consistently escaped.
I think addMeta() should specify that the values are always wikitext, and we should have a helper method somewhere to do the 'almost-escaping' correctly.