I mainly ride mountain bikes (on roads), so that might be my disconnect. The tire-width thing makes sense. I also hadn't realized before this thread that road bikes don't usually use disc brakes. Seems odd.
Disc brakes aren't always used on road bikes, because their advantages aren't excercised much - you generally don't brake so much that heat is a problem; and wheel bending isn't a problem since unlike mountain bikes, you aren't expected to hit stuff that might bend a wheel and continue riding.
For wet conditions, disc brakes would have an advantage on road bikes for others, I'm not sure.
