Tenkar's latest blog post talks about his wife asking why he needs two copies of certain books and to a point I can understand her point. I myself am a big proponent of own two copies of a certain books. I own two (or more) copies of Swords & Wizardry Complete, Swords & Wizardry WhiteBox, D&D Rules Cyclopedia, The Star Wars Roleplaying Games (West End Games; two 1987 First Edition copies and one REUP), Labyrinth Lord (3 copies of revised core, 1 AEC), The One Ring, Rocket Age, World of Darkness core (the newer version), Vampire: The Masquerade, Werewolf: The Apocalypse, Changeling: The Dreaming and Changeling: The Lost. Probably a few other games too, but that's all I can think of off the top of my head.
The thing is, in spite of claims otherwise this isn't gamer-hoarding and I'm not suffering from dragon-sickness. The games that I have in the past, and will in the future, GM/DM/Storyteller regularly. I prefer to keep two copies on hand because most of my players aren't as into the hobby as I am and are not as likely to take the time or money to persue these books. So, I have a table copy, and a copy for my use. It just makes things easier to say "Oh, that rule's on page 125, see it?" than to take the book, flip through, find the rule, explain it, then take the book back.
Not only does it speed up play, but it also promotes players learning the rules on their own because when they read it, the rule might be easier to retain. Also, shit happens and if someone spills a drink on a copy then I'm not up a creek.
Another useful aspect of having multiple copies (at least in the case of Labyrinth Lord, Swords & Wizardry, The One Ring and Rocket Age) is that when I'm writing I can have two of the same book open at the same time for easy cross-reference while writing.
So yeah, multiple copies isn't gamer hoarding. It isn't. I swear. I don't have a problem. I can quit any time I want! Now if you'll excuse me, I see my wife eying my gaming collection while standing beside an empty box. I need to go.