ASL is open to everyone. After all, it stands for American Sign Language, not American Deaf Sign Language.
I can think of a good use for signed languages for hearing people to use between themselves. What if one of them had a cold, sore throat or some other condition that made them hoarse or otherwise made it hard to speak? If they happened to know a signed language, they'd still be able to communicate in places that don't have paper or computers. I wonder what the oralists would think of that.
Another use would be to communicate over long distances like between floors at an atrium without yelling. A really cool use would be to give signed tours of coral reefs on scuba dives.
Check out this
page for examples of sign language usage by hearing people. Take that oralists! :P