Lots of TW's through the years have worked just fine without the oil cooler. The engineers who put the TW design together clearly accounted for operating heat in the specifications!
On the other hand, I'm sure a lot of TW's have died due to overheating. There is a big oil cooler thread here (I'm too lazy to search for it right now) where various folks give readings about heat with and without a cooler.
As I figure it, especially in the summer heat, the oil in a TW is running at a temperature that is very close or even over the point where oil degrades. The oil cooler can bring this down 15-30 degrees which makes the oil happy, which makes my TW happy, which makes me happy!
For the money, I think an oil cooler is cheap insurance for longevity of the TW.