I have always done the full distance as a training run. Only started running at the age of 49 and did my first marathon in my 50th year (London)
Why did I do it? It seemed odd to me that the first time you would experience the full distance would be in the actual race. All the advice said not to do it, but I was out on one of my long training runs (20 miles) and I just decided that I wanted to do the full 26.2.
Advantages - if you have never done a marathon how do you know what pace to set? Having done it is a massive boost in confidence and a benchmark for the actual race. It also allows you to get your fuelling sorted.
Disadvantages - none as long as you don't do it too close to race date. For London I usually do it some time in early Feb.
Note: It should just be seen as your weekly long run in your training schedule - if it takes too much out of you so that you can't complete your schedule for that week you shouldn't do it.
For my first London I was aiming for sub 4. I did my training 26.2 on a much hillier course in 03:23 and the race in 03:12. The training time gave me the confidence to go for it on race day