why not train all it is easier
There are only two training programs that train your whole team - stamina and free throws. While I can see how either would be useful every now and then, neither are for long-term training.
Maybe you meant 3 position training? This is the training program that will net the least amount of money per week that you train. Well, some people say you can make money this way by skill trading (buying a player off the transfer list, giving him 2-3 pops, then selling him), but I suspect that's not what you meant. If you go this route, you will never train a top level player.
I can see the reason for training 2 positions, it is easier to balance your minutes and still makes you a good amount of money. Plus you can usually field a more competitive line-up. Three position training or doing team training all the time is just a waste.
Run of the Mill Canadian Manager