You were taught wrong. In many states have your flashers on is illegal unless you are stopped. In VA where I live you can have them on only if going below 30 mph.
Most states say you can use them to indicate a road hazard. I don't really consider rain to be a road hazard worthy of being indicated. If you don't know it's raining you shouldn't be driving.
I agree with the OP, flasher on the road, especially in adverse conditions make things more dangerous, it's distracting and makes it hard to tell who's moving, who's not, and who's going where.