What the runners receiving this advice usually don't know is that it is part of a sports-drink marketing program. Many of the coaches and nutritionists who encourage runners to "pre-hydrate as much as possible" before races received their own education on pre-exercise hydration from sources such as the 1996 position statement on exercise and hydration authored by the American College of Sports Medicine. The ACSM is the world's largest and most respected exercise science institution. At the time when this position statement was published, the ACSM was backed by two "Platinum Level" corporate sponsors: Gatorade and the Gatorade Sports Science Institute.
The mere fact that the advice to load up on fluid before races comes through an interested party does not guarantee that it is bad advice—but, in fact, it is bad advice. The only thing you'll accomplish by drinking excessively before a race is to increase the number of bathroom trips you need to make before and during the race. Humans are not camels. Our bodies have minimal capacity to store extra fluid. Any fluid you take in beyond the amount that is required to attain "euhydration" (or normal hydration) will go straight to your bladder. Unless you have allowed yourself to become severely dehydrated over the course of the day and night before your race, you will not need much fluid to attain euhydration on race morning. Twelve to 16 ounces consumed between the time you wake up and one hour before the start of your race should do the trick. If you urinate at least once after your initial visit to the bathroom upon waking up, consider your mission accomplished.
Tim Noakes, author of Waterlogged: The Serious Problem of Overhydration in Endurance Sports, advises athletes to stop drinking two hours before the start of a race. I prefer to wait until one hour prior to the gun to stop drinking. I find the two-hour cutoff to be impractical, as I'm often just waking up two hours before race time, and as long as I don't drink too much before, then the one-hour cutoff suffices to eliminate the need for mid-race bathroom breaks. Many runners who don't realize that they are not camels continue to drink until minutes before the race begins, a behavior that makes about as much sense as drinking a liter of water just before setting out on a long drive toward a destination you're in a hurry to reach.
You can use just about anything you want to hydrate on race morning: water, juice, a sports drink, or some sort of liquid meal. Just save the beer until after you finish!
For more hydration details, check out The New Rules of Marathon and Half-Marathon Nutrition.marathon.