There is no blanket, perfect answer.The TOS are subject to interpretation and you cannot make too broad of statements about the rules because spammers change tactics and then we have to adjust (although, in context, it's sometimes best to make a statement directed at a particular user that is clear and unambiguous). If you report blogs that are suspected as spammy, we will review them and decide if it's allowed. Without seeing the blog, I can't make any definitive statement that will always be true.
In the case of the user in the thread you mentioned. The first blog on their account is: http://earnmoneyfastonline.wordpress.com/ I can tell by just looking at the URL that it won't be allowed. :) See http://en.wordpress.com/types-of-blogs/ and http://en.forums.wordpress.com/topic/affiliate-link
The rule is, if the content in your blog is good well-written, real, original, not-duplicated, not designed for SEO only, not designed to generate affiliate money then you don't need to worry.