I descended Angeles Crest in the pouring rain on my Cannondale road bike and the pads were ground way down, stopping power was not good, and that black aluminum slurry got all lover everything.
There are some steep drops around here where I sometimes wish I had more brake power even when it's dry. I'm 200# and discs on a road bike might be worth it to me, although I only have them on the mtb so far.