First, it's hard to get the same range on a double that you can get on a triple. Second, when trying to get close to the same range on a double that you have on a triple, your gears are going to be bigger jumps (which some people don't mind, but I really do).

Some people tour on single speed bicycles and like them. There aren't really any (real) disadvantages to them. If it were me, I'd get a triple.