All 3 of my vents are powered, with the one in the bathroom having a rain-sensor built it.
I put covers on the 2 without the sensor, figuring the one with it didn't need one. Well, that worked fine until I found myself in a place where it rained for 3 days straight, and I couldn't get the bathroom vent to open when I took a shower because the sensor was wet.
So now I have covers over all 3 vents, and everything works great.
Since all of my vents are made by Fan-Tastic Vent, I went with their UltraBreeze covers with the bug guards: