I use a radiator cover because it will take 20 minutes for the truck to warm up at times (yes while driving), if it is really cold out, so at times it IS necessary, for me anyway.
Also, the t-stat still dosen't affect the fact that these engines will cool down at an idle.
I tried plowing w/o the cover today, and still got temps of 210, but they fell quickly and never went over that mark. From now on I'll just pull out the cover when plowing.