Theoretically, you can only stop as fast as your front brake can stop you and the rear brake is not "necessary". Practically, it makes a lot of sense to use both since we probably don't all have perfectly finely honed skills with front braking only. I also use both because I'm cheap and want to distribute wear to both brake pads as evenly as possible.

I dont think most adults need to worry as much as they do about an endo from braking with the front brake. That's actually fairly difficult to do unless you are a serious lightweight and/or going down a steep grade.