I may have to concede irrationality on this point.
As any reader I have is likely aware, Marginal Revolution is a pretty successful blog.
One of the contributors, Tyler Cowen, has a book published: Discover Your Inner Economist. To promote sales of this tome, he has (i) offered access to a secret blog for those pre-ordering and (ii) offered customized podcasts to those pre-ordering. There may be other promotions, I haven't followed MR closely enough for a sufficiently long period of time to tell.
Either way, this has actually dissuaded me from purchasing the book, and I have resolved to read it at Chapters or some equally tolerant book store. I do not object to the price, but something in me twinges when people start restricting the flow of previously free content in order to compel fans to purchase the real thing.
To invoke another example, one of my favourite webcomics is The Order of the Stick. The author does not offer all comics strips for free on his site. Thus, I downloaded a pirated copy of the strips I could not access for free.
I'm not guaranteeing I would purchase if all content were made free online, but I just don't like splitting the community through 'premium members' or whatnot. Obviously, I may be in the vast minority when I purchase things that are freely given away, but am less likely to purchase those that aren't.