I have the Maxxair 8500 with remote (required, due to vertically challenged DW) and the rain sensor. Even with the rain sensor, you'll still want a cover, and here is why:
The fan is running and the lid is open, and it begins to rain, so the fan shuts off and the lid closes. So far so good. Then it stops raining, and you want to open the vent. But the rain sensor is wet and thinks it is still raining, and will not allow the lid to open. So you press the button that shuts the sensor off. Now the lid opens, even with the water on the sensor. The sensor remains off, even after you turn the fan off and close the lid. Then next time you use it, the sensor is still off. Then it rains... and the lid stays open, because you told it to.
