We have plenty of people with Neo’s that have added a rocker plate. They notice a real improvement in motion and usually in comfort as a result. The flex built into the Neo is better than nothing, but it is pretty minimal when compared to a more functional rocker plate.
The fact that a RP allows that subtle shift from side to side, it replicates what we experience on the road. It changes the loading on the sit bones and related tissue with every bit of motion. I find it removes that tendency to grind those parts with no change in loading.
Overall, saddle comfort is the driving reason people build a rocker plate. Most of the people I have seen add one, say that the RP has noticeably increase the seated comfort on the bike, especially when seated. Very few say there is no difference.