I actually did a blind test where I put squeezable margarine on 3 sets of ribs and left the other 3 as is. I had a bunch of friends over and had them taste them both. It was 7 to 2 in favor of no margarine. Most felt it had lowered how much smoke the ribs took in and had made the rub seem milder as well. Those that did like it said they could stay taste just a hint of the butter flavor and liked it.
I believe the logic behind it is to help with the texture and sheen of the ribs but even when I do backyard comps I will be skipping it myself.