I think it would be cool to have real automatic transmissions simulated, I wouldn't use them, but I think as a physics demonstration it would be pretty cool.
Yes, I realize that automatic shifting is in there as a driver aid right now, but it's very unrealistic, the only cars I can think of that shift like that are a few Ferraris and Maseratis with the F1 transmission, and SMG equipped BMWs.
Ball-Bearing-Turbo is correct, while enthusiasts would obviously buy the car in manual transmission form, all of the street cars in LFS save the LX4 and LX6 would be available with an auto.
All this being said, I'm guessing it would be costly and time consuming to develop a real Auto trasmission, and I doubt it will ever make it in the game.