I guess it depends on the nature of why you left. For instance: I would not post such a thing (or at least I would do so anonymously) if I had a very negative experience with a company.

However, if the reason I left was simply one of cultural differences, or some other not-quite-a-fit scenario, I would certainly have no qualms attaching my name to it. Like I said, that's often something you're asked about in an interview anyway.