Actually one button mode has been in for a while. I could not find the thread but here are the highlights.
The "one button" mode was added in to help with people with disabilities actually be able to play MLB The Show. I also believe this was a factor in the design decisions of some of the manage only mode crowd. It made it much easier for more people to be able to enjoy the game and experience the sport that they love as much as the rest of us do.
I, personally, think this is an awesome addition to The Show. Adding features and game modes that make the game more accessible and entertaining for more people can never be considered a bad thing.
Before just dubbing something as a bad idea or a poor design choice, you might want to think about how these choices and modes could be used by everybody or increase the overall quality of the product as whole.
But that's just MY .02