I'm certainly not one who believes the generator needs to be run every month. Mine is either stored in very cold weather where it makes no sense to try and start it, or it's stored a couple thousand miles away. But having said that, I think it should be run more than 2.5 hours a year to warm up the components, dry out moisture, blow out the dust, keep the gaskets from drying out so they don't leak, to not loose the fuel prime, keep the contacts in the starter working, etc. The moisture in the air can go back up the bare metal in the muffler rusting it out if isn't heated and dried out occasionally.
Yes, my 7.5 Onan sits built into the front end of my DP where it's protected from rain and snow, but I wouldn't call it weather proof - or at least not moisture and dust proof.
Bill