Technically, not intentionally. I see the book (usually a system) and say "Hey, that looks like fun." I buy the book. I know my group will never play anything other than D&D, and I'll never actually play the game I just bought. But that doesn't mean I don't WANT to.
All the time. Though I'm looking forward to start my first Ghostwalk campaign, almost two years after I purchased the book. I'd like to play Castle Falkenstein too; I own the book and love it, but never got a chance to play.
Never. Ever.
All my gaming purchases affect my way of running a game and its contents, one way or another. Some aren't directly played or used, but they always influence my games somehow, by giving ideas or striking me as "something I really would not enjoy".
All the time, although I rationalize by telling myself, "Hmm... I might want to use this for reference sometime!" About 20 or 30 GURPS sourcebooks alone made their way into my collection like that.