Right now, you pay the same price for a roundtrip award and a one way award booked with Delta miles. Obviously, under current rules, booking a one way award is foolish.
Starting January 1, 2015, we will be able to book one way Delta awards for half the price of roundtrips. (We lose the ability to book stopovers on Delta awards the same day.)
To me the ability to book one way awards is especially valuable with Delta miles.