well i had one with the window and i am not sure about other smokers but after i started smoking the meat i cant see thru the window !!!
so i sold that unit and got one with out the window , since i do smoke all year and even in the winter i have noticed the one i have with no window my temps stay up longer and don't go up and down with the cold wind and here in Oklahoma ....
I can easily believe that the window would get smoked up part way through one smoke.
Your point about heat loss is very good. The MES is well insulated, but the glass window would cause a heat loss -- just like a single pane window on a house. That makes it a distince disadvantage in my mind, and one I had not thought about before.