I ship thousands of cards per year. I never used decoys before 2010. But in late 2010 I had a couple of buyers complain about creases. I did some of my own testing, and in my experience it is very easy for a jersey/patch card to become creased during shipping in a bubble mailer if decoys are not used. Often the damage is very subtle so I don't think many buyers even notice it. But the fact remains that even a slight amount of pressure, from a dime sized object (a thumb, for example), can create a crease in the jersey/patch window. A hand holding a bubble mailer tightly so that they don't drop the bubble mailer, like a mailman for example, is a very likely way to cause a crease in a jersey/patch card. Some jersey/patch cards are more susceptible than others.
When a card is shipped with decoy's on either side of the top loader in a team bag, the same amount of pressure that would crease a non-decoyed card will not crease the decoyed card. This is because the decoy spreads out the pressure over a larger area, and thus the amount of force exerted onto the jersey/patch window is minimal, if any at all, and thus it will be undamaged during normal transit.
Of course there will always be times when no amount of decoys will save a card from damage during shipping. But in my experience, I would never ship a jersey/patch card without a decoy, on both sides of the top loader.
I use cut up pieces of cardboard from the cases I buy, cut to the size of a card. Cardboard, for one, is cheap. It also is stiff enough to spread out the pressure across the entire top loader, however it also very light so it hardly adds to shipping at all. The only downside is the time it takes to cut up the cardboard. But that is minimal if you buy a large paper cutter. It takes me less than 10 minutes to cut up enough decoys to ship 20-30 cards.
Hope this helps.