It seems like you can get the same range out of a double as a triple, unless I'm missing something. You just set up a double with the same inner and outer rings of the triple, and drop the middle ring. What you lose is the amount of options in the middle. If you like to find tune your gearing for a particular cadence, or if you like to have steady increments to shift through, a triple will get you there. If you don't care as much and just want to make sure you have a low gear for hills and a high gear for speed with a few options in between, a double will do the trick.