I have used it to piece duvet covers where the customer didn't want a batting and backing - they just wanted patchwork, and the serger made a good durable finished edge. It really was fast, but it's much harder to match points and corners correctly. It works for tablecloths, too, or patchwork aprons and skirts - anything that you don't want to put a back on.
I have also used it when we made charity quilts. Nursing homes and orphanages love the washability of doubleknit quilts - they can wash them in the super-hot water they require for sanitary purposes and they never fade or shrink or get holes even if they have to be bleached. We serged the tops and used medium loft battings and whatever we had for backings. Some were tied, but machine quilting made them more sturdy.