Just how resilient during a down pour of rain is the iPhone 6s? Right now, I keep a waterproof case handy just in case there is any precipitation. Is that necessary?
Not very as its not considered waterproof/water resistant. Iphone 7 or above includes the water resistance rating IP67 of up to 1 meter at 30 minutes. I wouldn't have the phone out in the open when its raining.