I strongly recommend bikes for touring be equipped with the broadest range of gears available. A heavy bike is more subject to momentum than a lighter one. In order to make the most of a downhill and make it up the big hills, it's nice to have a range from as low as most mountain bikes to as high as most road bikes.
Including myself, four full panniers, and the twins in their trailer, I estimate my bike weighs about 300 pounds. So far, I've had to replace three broken spokes, but it handles very nicely. On my first Bike4Peace ride, the tandem weighed about 450, but with two people driving it (thanks, Ananda). We broke a lot of spokes and, because my front rack was too high, I fought it more. I'm glad I built up my legs last summer and we're not riding such long distances on this journey.
The bicyclist has great incentive to "unstuff." We become painfully aware of every ounce we carry. Of course, the easiest place to shave a few pounds is usually the rider, and several of us are experiencing that process as we ride. And the most important place to save weight is the rotating parts, because you have to push them around more. Tourists are well served by small SPD pedals, but want to shy away from wheels that are too flimsy.
As a gearhead, I am often challenged by the need to reduce stuff. It's tricky to balance the desire to always have the appropriate item for the task at hand, but at the same time to minimize the weight I haul. One learns to greatly appreciate durable flexible tools, especially small, lightweight ones. I've gathered a few of which I'm quite fond. The more frequently I use one, the more attached I become.
Still, part of the challenge is determining one's role. On the last ride, videographer was a primary role for me. It didn't take long on this ride for me to decide to mail my camera equipment to my sister to make extra space for diapers. Babies naturally take priority. Two minutes of the girls singing and laughing back there is easily worth hauling them and their stuff all day. Still, I'd love to find lighter, less bulky and more effective diapers...