Answer: when it doesn’t provide a feed. I found John Mayer’s blog this morning via photomatt and tried to sign up for the feed. Except his “blog” doesn’t have one. I really can’t see the reason for not providing a blog feed in RSS or Atom. The whole purpose of feeds is so I know when there has been an update and will visit the site/read the entry. If needs be why not provide a non-full text feed like Zach Braff? I really enjoy John’s music (especially new album Continuum) but producing a blog without a feed is like having a ninja without a sword.
Update: It turns out that he does indeed have an RSS feed. Chris McElligott comments on photomatt’s entry that it’s available here: http://johnmayer.com/rss/rss.xml Question is though, how are you to find that? The reference is buried in the header of the page. It appears as if the ninja is hiding the sword rather than having none at all.