Actually, unless it's really overheated, the fan isn't preventing much dew by the heat it produces. The fan is forcing convection, which offsets the heat lost due to radiative heat loss to the sky with respect to the convective heat transfer from the surrounding air. Remember that the if there isn't fog, the air temperature is above the dew point temperature. Without the fan, radiative heat flow from the corrector surface will be larger than the natural convective heat flow from the air (warmer) to the corrector (colder,) which is why the front of the corrector can get pretty cold (approaching 6C below ambient under some conditions.) When you force convection with a fan, the heat transfer from the air to the corrector goes way up. That helps to keep the front surface of the glass at a temperature much closer to the surrounding air--and likely a bit above the dew point. Fortunately, it doesn't take very much air flow to greatly increase the coefficient of convective heat transfer. Unfortunately, a fan won't work as well as actively heating components when it's really humid.
The downside of the fan is potential vibration and image degradation due to turbulence--but that's usually better than getting totally shut down because of dew!
Yes I think the main effect from the fan is that the motion of the air prevents water molecules from sticking and building up. Heat from the fan is minimal, but I think it helps more than it hurts.
Fan vibration can be minimized by choosing a good fan, and mounting it on a somewhat flexible attachment (not too rigid). Most of the recent high-end fans incorporate some sort of magnetic levitation of the fan rotor that minimizes friction and vibration (Adda HYPRO, Sunon MAGLEV, etc. bearings). Using thin metal or flexible plastic for the mounting also helps. If you have room (and you probably wont in this particular dew shield application) -- the fan can be mounted on springs, or rubber bands, or rubber grommets. You can further reduce vibration by balancing the fan blades -- running tests and adding tiny bits of tape to the blades -- sort of trial and error. A speed control can also be used to avoid any mechanical resonances. If done with some care, there should be zero vibration.
In practice I've not seen much image degradation from turbulence. My philosophy it that it is probably better to have the air at the corrector well-mixed, vs. some slow convection driven by temperature differences. As a practical matter, you also have to weigh this against the air warm currents from a dew heater strap (less heat needed with fan running) or using one of these 100 Watt heat guns every 15 minutes (large temperature over-shoot), etc.