A recent edition of the UG (maybe for
Disneyland?) addressed this in response to a frequently asked question: "If you publish these touring plans, will they no longer work?" The response had some statistics on how few people bought *any* guidebook *at all* (including the nice eye candy but otherwise relatively worthless Birnbaum) before visiting. It was shockingly small. You'd think that for a vacation on which you are spending thousands of dollars you'd spend at least a little bit of time and money figuring out how to get some value out of that. Apparently not.
You know what's even more interesting? Those folks come home talking about what a great time *they* had, too! As exhibit A, I offer you my brother and his family. They went to Disneyland. For one day. Between Christmas and New Years in '03-'04. They arrived at 11:00. They left at 5:00. They had no plan. The did literally EVERYTHING WRONG YOU CAN POSSIBLY DO. They saw three attractions (and that *counts* a meet-n-greet with Pooh), plus the parade, and ate lunch.
And you know what? Despite spending nearly $150 just to get three people in to the place, they left feeling good about the visit, and they wanted to go back the next year. I bought them RideMax. They still arrived at 11:00 and left at 5:00, but at least that time they saw six attractions *plus* Pooh. (They really like Pooh.)
This year, they actually asked for my copy of UG/WDW before their trip there, and I bought them a brand-spankin' new copy. Mostly because I couldn't bear to part with mine.