I too questioned the addition of disk brakes and was told that they will last longer, offer better stopping power, and will still work properly if a rim gets bent while on tour.
I think the one valid point here is the stopping power in the rain, although not when dry. Our road tandem has mini-V rim brakes, and I can easily skid even the
front wheel on dry pavement with one finger on the lever. Bill McCready, president of Santana Cycles (the tandem maker) has done a ton of brake testing, and says the disc brakes are not as strong as good V brakes, they're heavier, and require a beefier (read: heavier) fork and chainstay. They have more wind resistance, and from other posts on the tandem forum, I see the pads don't last even one-fifth as long as our mini-V rim brake pads have lasted. The discs do however make for longer rim life if you ride in wet, sandy conditions a lot, since that makes the rim brakes grind the rims down.
As far as stopping power change in water, our tandem's stopping ability does not seem to suffer at all in the rain, even when we were going through puddles deep enough to completely cover the rim at the bottom. That seems unusual though. I was very surprised. I suppose it's because of the machined rim braking surfaces and the right brake-pad compound.
As far as disc brakes "still working properly if a rim gets bent while on tour," the other side of that is that in a long descent, the heat can warp the disc and make it useless, according to actual results in Bill McCready's tests.
Unless you're riding in the rain a lot, it seems from my reading that the main reason people get disc brakes on bikes is that they look cool. Someone on the tandem forum said the disc brakes on motorcycles show their superior performance; to which one of the tandem dealers responded that his motorcycle's disc brake system weighed 40 pounds even though the motorcycle can use the engine to hold your speed down on descents. "Is that what you want?" he asked.
Don't get me wrong-- I think for most people they'll still do the job just fine on a single bike; but from all my reading, I have to say they're not generally superior.
This message was edited by whittierider on 10-30-08 @ 9:48 PM