There's nothing wrong with Maltodextrin. Malto will cause an insulin spike similiar to what dextrose will; thats exactly what you want during and most importantly post workout to replenish your muscles with glycogen and get the protein to them (this is why post workout shakes have protein and a simple carb such as malto or dextrose).
Now if you took a mixture of protein and malto like majority of mass gainers are, the insulin spike it creates will cause it to be stored as energy unless your body is in a state like it is during/after training.
Oats on the other hand are a complex carb, unlike a simple one the body must break them down into simple sugars which takes time. This will cause a steady insulin release and blood sugar level, which is what you want except for post workout.
Heres an idea of what I'm on about:
Mid-Morning Shake - 2 Scoops of WPC with 3/4 cup of powdered oats (provides protein and a slow acting carb)
Post-Workout Shake - 2 Scoops of WPC with 25g of Dextrose (provides protein and a fast acting carb, restores muscle glycogen levels and delievers protein to the muscles)
Before Bed Shake - 2 Scoops of WPC with Milk (milk protein is cassein, along with the fat content this will slow the digestion of the protein to last me through the night).
Insulin can act either Anabolic or as a Fat Storer, post workout it will act anabolic and therefore a spike will be useful, however if your lazing around or your body was in a normal state an insulin spike will act as a fat storrer storing carbs for energy as fat.
