I like the full-length raps you get from doubles, and they are the only way to roll when climbing in a party of three. But, I always use them like twins - I have yet to find any real-world advantage for the complication of traditional double-rope technique.
I have been toying with the idea of a 6mm retrieval line and a single rope for two-person climbing where I want long rappels. There are downsides, but it seems like a nice middle ground - similar weight, but less tangle and complication when climbing.