score:5
Is it usual, no. Does it happen, yes.
Baggage travels from the terminal to plane side in a variety of conveyances, some which are covered and protected from rain, some which are not. And unfortunately, there is really no covered area plane side where they could move the uncovered baggage cart to if it starts raining. While you may see the ramp staff hide under the wing during rains, safety rules often prohibit the tugs and baggage carts from going under the wing (especially on smaller planes that would have hand loading).
For the most part you probably need not worry about it, but it never hurts to consider how you are flying and through which cities. If your flights involve smaller aircraft (737, 757, A320, RJ, prop planes, etc) and you are flying through a city where frequent showers occur, then protecting things inside your bag (especially those that might be damaged by water) might be warranted. Otherwise, I wouldn't be too concerned.