The standard cover doesn't fit a dual orthomode feed. So I don't cover my LNBs either. Besides that the LNBF you are covering has a very nice heatsink to keep it cool in the sun. And the primary reason for covering would be to keep the heat down, not to seal out the wet weather.
My C-band dishes have voltage-switched LNB's and I don't use covers on any of them. I think the main reason for using covers was to keep rain, ice and snow off of the LNB's that had mechanical polarity switching. My reasons for not using a cover are these:
1. Stinging insects that seem to think feedhorn covers were made just for them
2. Heat dissipation is impeded in the summer because there is no ventilation
3. In the winter a feedhorn cover offers a much bigger target for snow collection, which can weigh down a buttonhook support and in any case can block some of the signal to the dish.
The main thing is to use watertight connectors, with weather protection boots if you have them. I also keep a tube of genuine 100% silicone caulk (not the fake stuff with "acryllic" in the name, that stuff is crap) around and put a nice coating of it over any remaining exposed metal (the entire connector if you aren't using a boot, or at both ends of the boot). The nice thing about genuine silicone is it is easily removed if you ever need to change out the wire or the LNB, yet it offers absolute protection from the elements. Just be careful not to get it on any heat sinks. Silicone is also handy if you have a feedhorn opening cover (the round plastic part that goes into the open end of the LNB's feedhorn) that wants to keep falling out, just three or four small dabs of silicone on the edge will glue it into place.
I haven't used a cover for years. Did have one LNBF suffer from moisture intrusion but I was able to fix it, still using it. Think it's better without, not so much weight on the support tubes/button hook, and better cooling. Works great at -40 in the winter... but I am no longer using a polarotor feedhorn.
My avatar does not show it, because I took them off for the photo, but I use half-gallon plastic milk containers to cover the LNBFs. Just to protect them (LNBFs) from weather and solar radiation. Probably not a big problem, but solar radiation striking the LNBF surface will raise the temperature slightly. The cartons do not last more than a year, but they are easy to come by....
My 9' RS I always keep the cover on, but my 12' Conifer has no cover. When I got it, there was no cover for it. I have one in my garage that I keep intending to try and fit onto it, but haven't gotten around to it.
Either way seems to be fine, here. I myself prefer to have the covers on so it looks a bit neater.