Josh, that maybe 'good ' advice for some one who is only doing light duty four wheeling. But, ... the number one enemy of automatic transmissions is heat.
The second is dirt.
Yes, you can do moderate wheeling without an auxiliary cooler. I have done it many times before.
But if I know of a better way to do things and if I can afford it, then I do it.
The 727 was rebuilt for a grand wagoner. The reason it needed rebuilding was slow speed wheeling and snow plowing. Snow plowing is done in cold weather when one would not think of overheating the transmission. But at those slow speeds there is not very much air movement through the radiator. Therefore the heat builds up.
Also, for the most effective cooling, one should hook up the lines so that the fluid goes through the radiator first and then through the auxiliary cooler before returning to the transmission. If hooked up in the opposite fashion then one would be cooling through the auxiliary unit and then heating it in the radiator.
And for maintenance purposes just think of all four wheeling as severe duty usage.