I've put over 8000 miles on my '02 Ski-Doo Legend 700, mostly with the carb heat on. If the temperature approaches the upper 20's, I turn it off. I ran the sled in 35 degree weather one time with the carb heat on (by mistake - I just forgot to turn it off) and heat damaged one of the rubber boots. I don't like restricting airflow, so I leave the vents open all the time. However, its the intake air - not the free flowing air over the engine - that creates the freezing problem in the cabrurators. If the air is moist or wet (e.g., if you have snow around the intake vent behind the windshield), the carbs have a tendancy to freeze without carb heat. I saw this happen to a friend's sled once. The solution for his frozen carb was to get out a hair dryer and extension cord and blow warm air on it for about 45 minutes. If the air is dry, it really doesn't matter how cold it gets.