Does an Author-Blogger Need a Gimmick to Sell Books?

This is the question I raised on my personal blog.  Of course, every book needs a gimmick to sell. But in the case of bloggers who get book deals based on their online content -- will existing readers of a niche blog spend the money to buy a book derived from that blog?  If the book features some or all of the same content they can read online for free, what's the incentive for them to cough up cash for a print edition?

An interesting conversation is going on about this in the comments. Stop on by.