Having just one rotor means just that- only one rotor. A guy could ride with just a rear brake, and never even have a front, and if he never used the front anyway, would never know the difference... would it change his stopping ability? No... since 100% of your max braking force comes from the front wheel, you'd just slide into what you didn't want to hit.
But having just one front disk means that only one disk is absorbing the heat of your stopping. With integrated braking, a full-pull on the front lever yields only half the braking capacity to start with... so it's a skinny argument at best. I will note two things for absolute-positive-certain:
The front disk on my integrated, is discolored. It's doing ALL the work.
One of the best reasons to have dual disks, is to keep the forks under same load.
When you apply just one caliper, the fork slider on THAT side is being applied torque from the turning wheel being restrained by just one caliper. This deflects the tube. If you have a sensitive touch, you'll note that if it's a left hand caliper only, the front end will bind, twist to the left and shimmy as you approach adhesion limits. If you have BOTH sides with calipers, the forks will modulate much nicer, stay straight, and stable.
Long term result is that fork seals and sliders last longer, disks don't glaze over, pads wear slower, braking is more controlled, and steering under braking much more stable.
