I removed the viscous fan on my 200 tdi disco shortly after I got it in february 2009. Then used the disco to pull a lot of sail dinghys over the mountain from south of Norway to east of Norway, and back home again, I guess it weight 1-1.5 tonne. Then later that summer, I pulled a trailer with the equipment for a weeks scout camp up to the middle of Norway, still without a viscous fan. No problem. During the winter, while "driving" through some unplowed roads, with ~50-90 cm snow in front of the car (managed to get ~300 meters and back in a couple of hours), I noticed a red light at the dashboard. To get the most of power to the winch, I had turned of the fan inside the car, and was running the engine at ~3000 rpm. Then I bought an electrical fan, and thermostat. (This was in 2010). I had some long journeys with trailer in 2010 as well. Without any fan. The fan was mounted an connected before we went to Polen to participate in 31st Pomeranian.
Now I have a switch inside where I can choose between 3 modes. Thermostat controlled fan, fan always off or fan always on.
Schematics are here:
It works for me.
As other has said, you only need a fan if you are going slow, but still put a load on your engine. At least on a 200 tdi
// Finn-Arne, who is still moderated, and therefor a bit slow on the replies...