It's definitely worth the 'bulkiness', which isn't as bad as you might think it would be. There are some lower profile ones out there although I get the impression that they might limit the amount of opening you can achieve with the vent. I know the MaxAir covers allow almost a full open tilt on the vent.
Any of the vents will rattle if open while going down the highway if there isn't a cover over top of it. Mine always rattled if the wind was blowing the right way. The vent cover stops all of that.
Also, while I've never tried it, I suspect that the rain sensor will only close the vent if the rain is enough to trigger the sensor. If it's just a light mist, it might not trigger and I figure by the time it's raining enough for the sensor to kick in, rain is already coming in the open vent. I've always liked to keep things simple - another electronic gadget that I have no control over, well, I'd rather not.
Depending on the color of your vehicle, you might even be able to color co-ordinate the vent cover to match. If you've got something like a blue van, you might be able to use plastic paint to match it, although unless it's UV resistant it might fade over time.
