Please don't do it. The only thing these items do is compel people to pay for a game they don't know enough about yet. And those who don't let themselves be coerced into that won't get it in the end if they buy the game after reading some thorough reviews. It's not fair to anyone.
Of course one could argue that we're talking about id here and they've never disappointed anyone but it's a thing of principle. id has been here from the very beginning, setting new standards and simply ruling all the way. Please don't sink to the same level as EA, Valve, THQ and many others have. It's pathetic.
