This is a very "sensitive" subject :)
I took the decision to use HTML for posts and sanitize it (keep the dangerous stuff)... including the style attribute... It is a "whitelist security" approach and the idea was to avoid XSS attacks and styling spam
but probably it can be too limiting for some cases...
It should clean the styles but not the class, that means that (ideally) the site admin can decide which classes to allow.
That is one thing (1). There are several way to improve this and we should be really open for discussion on this (what about Markdown?).
Other thing (2) is that while I was trying to explain this, I tested and I found out that even using the "styles" drop down list from the tinymce the class get removed while sanitized... this is a bug (the other is not).
Sorry for the long reply... What did you mean? about the bug or the other?