It's needed because the torque converter generates lot's of heat, much more so under heavy load and high horsepower. Higher stall converters create more heat when they are operated under stall speed due to operating ineffeciencies, and normal driving tends to keep them under those stall speeds. If it weren't for the T/C, auto trans fluid would remain pretty cool. The shearing of the fluid due to normal converter oil flow between it's components during torque multiplication creates a lot of friction and heat. This is the biggest reason high performance and towing vehicles run big external coolers, the small cooler in the radiator gets maxed out easily.
I also run a B&M Super Cooler, mine is mounted closer to the grill, off of the radiator with metal brackets. My trans runs unbelievably cool too, I'm sure mine will be overcooling once it starts to get farther into fall. I'd add a thermostat if it was my DD, but since it is primarily warm weather driven I won't bother now.
