Ride any bike, now get off of it and ride something else. They're different in some way, some more than others. Your brain needs to adapt to the differences before you are used to the other bike's characteristics.
Now put the ability to change power levels on one bike, it always seemed like a bad idea to me...regardless of one's skill level, even in the rain!!!
If you need to start in C and get comfortable, then step up to B, then A, and that works for you then fine. Keep in mind you now have to re learn the bike for B, then again for A, and your brain has to keep up with the changes. You also have to be aware of the power level should you need to get out of harm's way quickly.
If you need A power and you're in C mode, the split second difference in throttle response can be enough to get you ran over.
The same holds true if you're used to C power, and you nail it in A mode and aren't ready for what's about to happen.
I also hear people say how good less power is in the rain, these people are putting everyone in the same box. If you NEED C mode to ride in the rain, then you also need a less powerful bike.
As said, all you need is good wrist control. Ride in A, all the time, no matter how fast or slow you are comfortable riding. It'll keep you out of trouble. Use your wrist to regulate the power, not a switch