If your AC compressor seizes (not uncommon), you can bypass it with this belt.
That whole "if it is fine at 100K, it is OK" is kind of silly (IMHO) - our "perfectly fine" AT at 120K needed to be rebuilt, and I believe that if it had more regular transmission fluid changes, and/or a cooler, it "may" have lasted longer - who really knows.
I can say that the design of the AT cooler (where the AT is "cooled" by the engine coolant) is great for around town driving - your tranny fluid gets to proper temp quite quickly, and stays at a "reasonable" temperature. Going up mountain passes, hauling all the gear for a trip, etc will really get that fluid heating up, shortening its life, and the life of your tranny.
Adding a cooler also lowers the water temp when you are climbing passes (since it is not dealing with the extremely hot AT fluid in the stock tranny cooler), which is a nice side benefit.