There has always been at least one PHB per household in my group, so it's never been an issue. I run homebrew, so there's no setting book. But, if I ran, say, Eberron, I'd actively discourage purchase of Eberron products because I want to have the best understanding of the setting in the group -- even though I could change stuff, I don't want someone making incorrect assumptions based on a book.
I wouldn't mind if some of my players bought the "Complete" books. Better yet, someone could get Unearthed Arcana. Still, I don't require, or even suggest, such purchases. To be honest, my only wish in this area is that when someone decided they did want to buy a book, they checked with me, first.
The player of the druid picked up Masters of the Wild last year, just before Complete Divine came out. I never much liked the 3.0 splatbooks and was eagerly awaiting the Complete books to replace them. So, he bought a book that I was actively looking to eliminate from my game. *sigh*