I have two windows on my enclosed. Love them. I can leave them open as we drive the sleds up the hill, to let out any errant gasoline fumes, with out having to leave the two top vents open, that add too wind drag and I'm afraid will break. I also leave them open during the summer, to promote natural air flow out the top vents and again, keep down the gasoline fumes. The windows on mine are dark tint too, so I don't get any sun light fading on my toys. Even though they are dark tinted, I get plenty of light inside the trailer to get dressed without my 12v lights on.
Down sides?... I would say cabinet or hook placement issues inside the trailer, if you are adding more of either. They are aluminum windows, and I have had them freeze shut/open if I get moisture in the track of the window. My rear window is mounted where the spare tire usually hangs, so I had to remount the spare under the trailer.