I have one, mother in law needs ta mod it fer my stack, so haven't used it yet. Been bout 5 months without, smoker don't look no worse fer wear yet. I'll use the cover when it's done. More personal choice then anythin I think.
I've used the Brinkmann cover on my SNPP since I got it 14 years ago. With using the cover and an annual coat of high temp stove paint, it's still lookin good. I also bought the firebox liner to help prevent burnout. My firebox is still solid. I do not leave the firebox full of ashes and I think this helps to extend the life as well.
My MES came with a good strong one. It sits on the patio next to the MES. Never have fully covered it yet. Seems I'm always doing something with the smoker, checking temps, smoking water or smoking meats.