I'm very keen on double-stranded knitting right now. Most recently, I designed a sweater for my husband, and I had a hard time finding the perfect yarn; I needed something not too stretchy/scratchy/delicate/fluffy/expensive. Solution: double strand a cotton acrylic blend worsted weight yarn with a merino nylon sock blend.
Double stranding is also a great way to use those very fine yarns. Combine them with another yarn, and you get an easier gauge, and a totally new colorway.