There is really no reason why you should eat a PB Sandwich unless you are trying to gain weight and bulk up. People recommend natural peanut butter because it is loaded with calories and it is a source of GOOD fat compared to regular peanut butter which is mainly hydrogenated oils. So people that are trying to bump their cals find it easy to get alot of calories in the form of a good fat while at the same time getting a little bit of protein. When I was trying to up my cals and get some additional fat in my diet I turned to a scoop of natural peanut butter in my late night shake (delicious btw).

In general, this meal is just a bad idea if your not bulking because you are combining fats and carbs. You really want to have meals which have protein+carbs or protein+fat for a general muscle building diet.