The con is that the disk brakes make the wheels very heavy. The hubs are heavy, the rims are heavy, and the rotors also add to the mass you're spinning. I worked really hard to get the weight down on the wheels when I had a custom set made. They're still several hundred grams heavier than my heaviest (1609g) rim brake wheels. Climbing up to Estes Park was a real chore with those heavy wheels. Part of it was me too, as a lifelong low-lander and flat-lander, but one of my other bikes with lighter wheels would have been so much easier on that climb.
Nope. People believe this, but it isn't true outside of the very tight requirements of racing. Take a look at
http://www.training4cyclists.com/about/ Adding 1.8kg of mass to a bike's frame added about a minute to a 50 minute hill stage, and a minute and half if added to the rims. If added to the centre of the wheels then weight will behave as if added to the frame rather than the rim. So the extra work caused by discs is minute and beneath the limit in normal performance variation.