I read somewhere that in England, when you order Dahlias - what you receive usually is a rooted cutting, not a tuber. I believe that there is at least one Dahlia site here that also mostly sends rooted cuttings.
You can get a lot more plants if you take several cutting from each tuber. I mostly plant only one or two of a kind, so there isn't any motivation to do a lot of cuttings, but if you get good at it, then it would certainly be a way to have lots to trade or sell.
