The single leg Romanian deadlift is a fringe lift, but definitely has some usefulness. It isn't going to directly improve strength much, but should increase your balance, coordination, and mobility. Beyond the inherent value of improving these areas of your fitness, increased balance, coordination and mobility should help to improve your major lifts (squats/deadlifts/c&j/etc). Does this mean single leg Romanian deadlifts are a significant part of my programming? No, but they are worth throwing in every once in a while.