Free Dining was first used only during the slow periods to get more people to come and that is what it is going back to again seemingly. I always went in Septmeber however never had FD as we own DVC. before free dining was offered I could eat wherever I wanted when I wanted and most rides had a short wait. With Free dining most rides have a short wait and even shorter at peak dining times I can still eat where I want just now eating TS for lunch and CS at night. To answer you main question yes the parks will likely have more people but the DP takes a lot away from the lines as people try to maximize the DP and spent a lot of time eating.