Smoothies were originally designed for rock work on the bigger loaders. Idea was that with no lugs or tread, rocks would not have any one spot to dig in and tear out a chunk of rubber. With large loaders (980 up), traction in rock is not a big consideration as the weight of the machine, used properly, will supply all the traction needed. They don't work worth a tinker's damn in mud or slick conditions. That's why people prefer a lugged tire or one with at least some tread pattern to it for all around working conditions underfoot.
Otherwise, because of the size and cost of the larger tires, owners want to get maximum wear out of them, which means taking them right down to the last tenth of treadwear. You don't want to go too far and ruin the tire for retreading considerations, and you do have to think about what kind of material you are working in. If that tread gets too thin, it doesn't take much to puncture it and that could ruin a tire for retreading too.
If the tire has some sections in it or other damage that takes it out of the running for the retread shop, then it gets put on the rear and becomes a "run out." That simply means it's used to destruction, after which it becomes tugboat bait.