The "recommended" way is to install babel for each project independently. Space is cheap, internet speeds are fast for many, and avoiding version issues outweighs the savings.

But you can install globally, so you'd install babel once and can use it in all projects that way.

Then throw in the possibility of different package managers and you get a crazy amount of freedom and choice. People get overwhelmed with "javascript fatigue" and I just don't get it. You don't need to do everything, but having the option to is amazing.