Yes...
Yes, a defroster is not a bad idea, however, if you are seeing it on the windshield, it is most likely happening at other areas on the airframe, typically on small radius curves and anything that protrudes into the flow.
It is truly AMAZING how fast the "freezing moisture" can accumulate, if the right conditions are present.
Some years ago a family friend flew a 182 on a cross country and encountered ICE, (there, I said it). Thankfully, he landed at his destination...with almost two inches on the airframe and a ball on the spinner the size of a softball. You may have read about it in the AOPA magazine.
Also, some years ago, my family was snowmobiling in the Northwoods of WI when we came upon a guy flagging us down in the middle of a trail. It turns out the King Air that he and his family were riding in had crashed in a clearcut area. We carried him to the nearest stop on the trail, only to find the Sheriff's department getting ready to walk a grid through the woods to find the downed airplane. Weather conditions, at the time? FREEZING FOG. That King Air was on about a 15 mile ILS final and didn't make it...the crash broke the airplane in three pieces. Amazingly, everyone, including the dog, onboard survived; the worst injury was the pilot's broken leg.
Point is, "freezing moisture" can and will accumulate, sometimes after than expected. Our RVs are wonderful airplanes but they are still no match for Mother Nature.
Be careful out there!