Some runners like to run the occasional official marathon event as a training run in preparation for another marathon. I guess I'm one of them.
There aren't all that many marathoners who perform marathon-length training runs period, let alone in a race environment. But some feel that running 26.2 prior to racing 26.2 gives them an extra bit of endurance that helps them finish stronger when they later attempt to cover the same distance in peak shape, as fast as possible. That's how I feel. Plus, there's a confidence-building benefit.
If you're game to do a marathon-length training run, you might as well do it in the context of an official event, in my view. It's more motivating and less boring, you get some practice with event logistics, early wake-ups, and all of that stuff, and of course you don't have to carry any fluid, as you must when you run solo for three-plus hours. The potential downside of running a marathon as a workout is that it's all too easy to get caught up, run too hard, and ruin your entire next week of training. I've seen it happen.
Several years ago I did the San Diego Marathon (now the Carlsbad Marathon) as a training run. I went a little too deep and was a wreck for the next three days, but after that I felt an immediate quantum improvement in my fitness level. A couple of weeks later I set a huge PR in a half-marathon, and I've been a believer in marathon training runs ever since. In fact, as soon as I publish this post I'm going to use Active.com to register for the Sacramento Cowtown Marathon, which takes place October 14, seven weeks before my peak marathon.
Remind me not to let myself get caught up!