There seems to be several variations of the Romanian DeadLift. The version that I use involves some knee extension at the top of the movement. Others are strictly a hip extension movement. In the latter case, you are correct when you say that the RDL is identical to the SBSL Deadlift.
is an article that compares the RDL and SLDL.
From another article
What Makes The RDL So Great?
Two words - intensity and functionality. First, the RDL allows a much higher intensity level (basically more weight can be used) than a leg curl does. Considering that hamstrings are made up of primarily fast twitch muscle fibers which are best trained with higher intensity levels, the RDL is one of the most effective hamstring exercises you can do.
Second, the RDL is also far more functional than leg curls. Sorry folks, but leg curls are not a functional exercise. Although it may seem like knee flexion is a big part of your every day activities like running and walking, a look at the true biomechanics of these activities shows that it is, in fact, hip extension that plays the major role in these activities.
Your knee simply flexes in order to reset the leg and start the locomotion movement again, and even there the momentum generated from the hip extension helps swing the lower leg back. Hip extension plays a huge role in several everyday activities, such as the above mentioned running and walking, not to mention jumping and biking. In addition, when you learn to bend over with a heavy weight in the gym while protecting your lower back, you have learned better body mechanics for use outside the gym as well.
Bottom line - hip extension is involved much more often in athletic activities as well as in everyday life than knee flexion. And hip extension allows you to use heavier weights than knee flexion, which leads to greater growth potential for the hamstrings. Knee flexion should be trained in a lifting routine to promote healthy functioning of the knee joint. But it should be trained after hip extension in most instances. People who are rehabbing their knees might switch the order, but that's the only exception that comes to mind.