My opinion on one step sparring: Our instructor uses one-step sparring as a drill to teach accuracy, control, and distance. It can be a useful tool if done correctly. We teach it only to our white belts. An issue that I have with them is taht our techniques are suppose to stop an inch away from our target. So our white belts begin to create a habit of striking out of range. Sometimes it stays with the student through the advanced ranks. I still have issues with range.
We also do non-contact sparring, for white belts. Where all techniques are to be one inch away from the target. Some white belts do this for 3-4 months, builds really bad habits. Not my favorite type of drill.