We've always tried to review all the games sent to us, but there are always some that fall through the cracks due to busy staff, insufficient man-power, shipping problems, technical problems, etc. PR reps understand the pressures of running a website and rarely say anything except to check up on a title and ask whether we plan to review it. Many review copies come with a letter and the phrase "for your consideration". We consider everything -- whether we can realistically review everything is another question.
My friends used to ask me why companies sent me free games. I always said that if I write something about that game and cause one other person to buy it, the publisher has already made enough profit to offset the cost of my free copy. That's probably not true if you factor in the PR rep's time, but the potential advantage is still obviously enticing enough that virtually every game publisher sends out review copies. It's one of the most cost-effective ways to market your game.