Past experience.
I think about my anime DVDs that I bought fresh and new that dropped in price and were reissued cheaper before I ever took the shrinkwrap off. I bought one ebook at full price that went to free before I had a chance to read it and I decided I'm going to wait until I'm ready to read before I buy anything.
On the freebie side, when I first started digital scrapbooking I downloaded oodles and oodles of free kits that I not only have never used, but now they are something I have to manage and I have so many that I don't even know what I have, much less expect to get around to using. Along the same vein, I don't want to end up with a bunch of books I won't read but have to manage their files.