There is another additional reason if you don't mind me sharing it also. A transmission needs a "Normal operating temperature" just as much as an engine does to be at peak efficiency. Imagine running one in the winter under very cold conditions without the radiator to help add heat to the trans and bring it up closer to operating temp. Tolerances are going to be too loose, and the fluid viscosity is going to cause all kinds of issues. So in some conditions adding extra heat to the trans from the engine is just as important as removing heat.

 

I don't mind you sharing at all. That secondary purpose likely won't matter much to neverenuff since he lives in AZ.

 

I live in Az also and as you say it is not as common, but my trans has gotten cold enough in the colder higher mountain areas that the trans started to act funny, I knew it was because it was too cold. It was the last thing I needed on a steep icy back road. I found I had to route from my external cooler back through the radiator in the winter, it does much much better in the cold now.

Just pointing out that in colder conditions it does indeed serve a purpose...

 

It certainly does. I actually have a cover that I will put over my aux cooler in the winter to allow the heating from the radiator exchanger to be more effective.