Certain TinyMCE plugins scan for all spans in the document and remove any that do not have classes (or a handful of other attributes). This is quite aggressive, and breaks the regex used to scan for MCE views when transforming them back to plain text. Adding a class ensures that any plugins that do so will not break MCE views.
see #21812, #21390.
git-svn-id: https://develop.svn.wordpress.org/trunk@22162 602fd350-edb4-49c9-b593-d223f7449a82