Odds are that chick had step in (clicker) bindings, and those are prone for popping loose. If for some crazy reason you still have clickers, then yes, get absolutely get a leash. But it's virtually impossible for two strap-in bindings to both break loose. And if you hit the ground hard enough for that to happen, you've probably got bigger problems than you board flying down the hill.
If you have a decent head on your shoulders, and know not to take off your board while on a slope where it has a chance of getting away from you, you can probably do without a lease. Also, if you set your board down, always put it across the fall line, and always BINDING-SIDE DOWN!!