I've had similar thoughts when it's not actually raining, but there's the slightest possibility of some light sprinkle coming over with limited warning before I can get landed.
No matter how itchy my control stick fingers are from lack of flight time, I generally err on the side of caution and choose not to fly in that instance, because one well placed rain drop - even a tiny one, that got past the props into the motor which is open on the top, could have either no effect, or catastrophic effect, and I'm not prepared to find out which it would be.
I have taken off from wet grass by using the case the quad comes in. I generally always take off from something, just to prevent water, dust, dirt, grass, whatever from being blown up onto the lens and spoiling the footage from the flight, or into the motors and one day causing major problems.
