All the grit, sand and wetness will reduce braking performance with rim brakes. But my point is that disc brakes works great in any type of environment, and will last forever comparing to replacing braking surface of the rim brakes - which is the rim itself. Some say that rim brakes/rim will last a very long time...well, I can go thru a set of very good quality wheel set ( Velocity ) in about 3 years. OEM sets never lasted more than 3k miles. Add few sets of brake pads, and in my case, the total will be around $500... On the other hand, my bike with disc brakes, never needed replacing brake pads and the wheel set, and it will be less than $50 if I need to replace the rotors and brake pads at some point.
I like to ride fast, I like to brake hard, and my bike with me and all the gear on it is way over 300 lbs. If you are a " skinny ass", your experience may be little different.