I think many different factors come into play here. When you are learning to fly a model airplane, you have to learn about basic aerodynamics. Turning the aircraft requires (at minimum) the coordination of ailerons and elevator. If you have some basic knowledge of how airplanes fly, you might know this. If you don't, you probably won't learn it until it's too late!

Some individuals might be able to learn to fly without the help of an instructor, but I contend that most cannot. With the investment of time and money involved in getting started in this hobby, why would anyone want to take that kind of risk? If nothing else, invest in the "1 Week To Solo" video.