Anytime you run a marathon, you want to start it well hydrated and minimize dehydration during the race by drinking throughout it. (It is not necessary to completely offset dehydration during a marathon, and for faster runners it's more or less impossible.) At the same time, if you're trying to finish the marathon as quickly as possible, you don't want to have to urinate during the marathon. It takes at least 20 seconds for a male runner who doesn't have to go too badly to make a porta-potty pit stop. Those 20 seconds could mean the difference between breaking your PR and not breaking it.
Naturally, the goals of starting the marathon fully hydrated and minimizing dehydration throughout the marathon and avoiding pit stops are potentially conflicting. The easiest way to ensure that you start a marathon fully hydrated, after all, is to guzzle a liter or so of water between the time you wake up in the morning and the time you start running. But if you do that, you will almost certainly have to urinate at some point during the marathon.
How do you accomplish both goals simultaneously? It's not so hard. Many runners overestimate the amount of fluid they need to consume on the morning of a marathon. If you hydrate properly ever day leading up to your marathon, you probably only need to consume 12 ounces of water, juice or a sports drink after you wake up to compensate for fluid lost through respiration during the night and urination first thing in the morning. There's nothing to be gained from drinking more.
After having to make pit stops in a couple of marathons, I tried a new tactic of ceasing to drink anything in the final 90 minutes before the race start. I usually urinate twice during those last 90 minutes, my body getting rid of the excess from my early morning hydration efforts. This assures me that I am starting the race well hydrated and with an empty bladder.
If you use this tactic, there is certainly no need to moderate your fluid intake during the marathon to aoid pit stops. You will lose more fluid throught perspiration that you can comfortably consume while running, so if you start with an empty bladder, it will stay fairly empty.