I would never take a dim view of people being introduced to backpacking by this book--you've got to get the idea from somewhere, right? I tell everybody that A Walk in the Woods is what gave me the idea to do the AT, and Bill Bryson was not exactly a textbook thru-hiker either (thanks to Baltimore Jack, the word "candy-ass" gets thrown about a lot when referring to him). But my point is that you can't just read Wild, think "Oh that sounds so much like me! I'm going to fix all my problems on the PCT this summer too!" and go out there without much other information about hiking. Well, you can, but you'd be in for a miserable time.
Again, I don't know if appreciable numbers of people are going to do that. But given how enormously popular this book is (it reached #1 overall on the bestseller list last July and is still hovering at #5 for nonfiction), and how it has such a convenient message of self-help and redemption, I wouldn't be surprised to see a lot of people out there who prepared for their hike almost entirely by reading Wild.