I don't think the benefit is so much the ultimate stopping power of disc (locking up is locking up) but rather the ease of modulation and lack of hand fatigue on long downhills. I suffer quite a bit in the winter on long logging road descents (braking for 7km or more) in the winter as I have poor circulation in my hands. It's a lot less squeezing with the discs.
As I said, I've been looking into disc braking this winter for my mtb but also have disc on my endurance/touring road bike. It's interesting how "expandable" a basic disc set up is.
For example. if you buy a basic 160mm rotor mechanical disc brake bike you can upgrade to 180mm rotors with a simple adapter. After that you can also swap the mechanical for hydraulic using the same mounts.
Not saying all bikes necessarily need that much braking power but its interesting to note how you can get into disc at the ground floor and upgrade quite a bit over time with the same platform.