There really are no advantages to V-brakes other than weight.
In a mountain bike application, a disc brake is actually more efficient than a V brake (Hayes Hydro's put out 3,200 PSI at the pistons, clamped on an 8" rotor, it takes almost zero time to stop). Heat build up doesn't make a difference. Disc brakes are fairly uneffected by high heat situations, most systems dissipate heat quickly due to rotor design and caliper design. V-Brakes, however, can glaze over and fade rather quickly due to the soft compound of the pads. Disc brakes have significantly more modulation because of the constant lever feel, significant amounts of power, and the reduction of effort. Disc brakes reduce hand-pump. They are uneffected by mud, rain, snow, slush, ice, anything you'll ever see on a trail will not effect the brakes. You replace the pads once every season or two, depending on the compound you are running. With V's you'll have to replace them very often (due to softer compounds.) You never worry about pitting out your rims. You never have to worry about your wheels becoming out of true and losing the effectiveness of the brakes. You don't need to worry about cable stretch (unless you are on mech Disc's). Nor housing compression. Maintainence is no more.
There are millions of reasons to use discs, and almost zero to use V's.
The front of touring bikes are good to have a disc on if you carry high loads on panniers. Avid makes levers for drop-bars on cyclocross frames
