Can you post up some pictures of the circuit boards 'n such?
I'm guessing the rain sensor works the same way it always has, just they changed the programming in the brain of the unit, changing how it responds to the sensor being triggered.
From an engineering standpoint, the new method is what I'd choose - I'm surprised to learn the old way could have the fan opening and closing repeatedly and frequently, hard on the motors.
rjxj's suggestion of a vent cover is the best solution. With a cover, you can keep circulating air even if it's raining.
