Given the enormous number of books that are published in the run of a year, I see no need to purchase bad books. This goes double for kids books: seriously, I’m going to have to read them hundreds of times, and there are fabulous books in abundance. There’s no excuse for buying books with a corporate author (other than, y’know, that whole peer pressure thing).
I am a HUGE user of the library, though, and I love reading (as do all of my kids). I tend to read a large number of books on a particular topic in rapid succession, so I am likely to compare offerings. (I am, however, on the waiting list at the library, so they might be a little out of date by the time I get to them.)