I live where it snows!!
I have stored outside in the past and always used a cover. Despite dire warnings on here I never experienced any damage to my paint or finish.
My thoughts are if you live in an area with snow, freezing rain, hail and all matter of frozen crud that falls from the sky, why wouldn't you protect your rig?
I will add that any cover I have purchased is completely destroyed after three years of harsh winter. I have tried both cheap and expensive and expensive might last a year longer. I learned my lesson and shop for cheap. The main feature you want is a soft inner surface next to your paint.
It snows here in November and it is normally April before we see our lawns again. Here is some practical advise. Do not cover if you plan to travel before the snow melts. Once that cover is buried under a couple of feet of snow on the roof you ain't getting it off until the snow melts!
Neighbors covered their 5th wheel trailer one year and planned a trip south in March. The snow accumulated on the cover and there was no getting it off. He wisely decided against going on the roof with a shovel fearing damage to vents, skylights etc. He tried putting heat inside the trailer and melting it off but it was too slow. He had to postpone his travels for a few weeks!
Covering is a twenty minute job although allow double that the first time. Have some help, a ladder, and a positive attitude. If you live in a windy area add extra ropes to secure it. Wash your rig really well before covering and when you uncover in the spring it is clean and ready to hit the road!
Don't forget wheel covers as well.
Safe Travels!