Keyboard and mouse programs would make the keyboard and mouse players have a better experience?

Mouse and keyboard does not require to be part of the insider program. Just plug a USB keyboard and mouse in.

For the following to work you must make the menu selections using your keyboard. If you use the controller to make your menu selections to start the game you will be placed in a room for those with a controller and your keyboard/mouse will be non functional
to make it fair to other players. To enter the room for keyboard and mouse users the game must be started by selecting the options using the keyboard/mouse. Hope this helps.

Mouse and keyboard support

Fortnite on Xbox One supports the ability to play with a mouse and keyboard. Note that when using a mouse and keyboard on Xbox One, you’ll be placed in a game with other users who also chose mouse and keyboard as their control option. To enable your setup
to use mouse and keyboard, see Mouse and Keyboard Support

Note If you’re on a child account, you’ll also need to enable settings for cross-network play to use a keyboard and mouse while playing
on Xbox One.

I agree that Mouse and Keyboard can be an unfair advantage. It's really up to the developer of the game to tweak it and make sure that game is balanced. That could be done by only allow Mouse and Keyboard on certain servers, or giving the Controller players
more auto assists features.

I've been a Console player mainly and I have never been able to get used to Keyboard and mouse. I personally would rather use a controller.

I believe that Microsoft, and the other console manufacturers, should support keyboard & mouse natively.

Currently people are using keyboard & mouse adaptors which make the playing field far more unlevel than native support would do. How? Let me explain.

When you are using a controller games give you a generous amount of aim assist. When using the adaptor you are gaining all the benefits of playing with a keyboard & mouse, as well as that generous amount of aim assist. This gives players a huge advantage over
others.

This has gained such attention that players on PC are now using the adaptors to make their systems think a keyboard & mouse is a controller, because again they get all the benefits of keyboard & mouse as well as the aim assist for controllers.

So native support for keyboard & mouse would eliminate the advantage people currently have when using an adaptor.

I don't think they should add mouse & keyboard support for Overwatch on Xbox because it would cause balancing issues. Players using mouse & keyboard would have a huge advantage.