Mr blade stainless steel leading edge will cope with the rain upto a point. Watch for pitting (and eventually pin-size holes) at outboard leading edge. Sacrificial polyurethane leading edge tape is used sometimes to extend the life of MR blades. Works well but will not survive long being continuously battered by heavy rain (e.g. tropics).
Although not ideal for power, your compressor blades are getting an unscheduled rinse.
Rain can play havoc with avionics. The integrity of cowling/ fairing seals is important to keep moisture away from the wiggly-amps and black boxes.
Unblocked drainholes in the transmission decking will allow the floods to drain harmlessly away. Unblocked drainholes in the lower fuselage and cowlings will prevent 'pooling' and corrosion.
Wipers? They are getting better on some of the newer helicopters, still always seem to be a bolt on after-thought to me.