The rules only state the maximum thickness of the fuselage and the forward part of the naccelles or fuselage, so having said that it does not mandate what is expected to be on the outside of the model or not on the outside. Technology moves on so we figure out better ways to building the models over time that go against the traditional ways of building a model. I haven't built a profile using the traditional 1/2" thick balsa with doublers for probably over 20 years. Nobody seem to notice or even care I just know that they are stiffer and lighter than the traditional way of building the model.
Frank Beatty once told me one time he was told he was building "Giant scale" because he made the model bigger to accommodate the engine, fuel tank and 3-line bellcrank. At that time the 3-line bellcrank was new and engines were getting bigger and they had separate fuel tanks. So builders and designers had to adapt to the new engines and could build bigger. Oh by the way the giant scale model was about a 45" span, keep in mind in the early days the models were quite small (maybe 30" maximum span) because the engines were smaller in those days.
I wonder what they would have thought of my 96" span B-29 with electric power and RC controls? The B-29 has all of the wiring hidden inside the hollow structure of the nacelles and fuselage to improve the appearance.
When glow replaced ignition engines modelers probably thought the world was coming to an end
When Four strokes came in people had to get use to the new technology
When electric became mainstream it caused people to re-think the configuration of the control line model
I have have not used the traditional ways of building, painting or controlling the throttle and other features for over 29 years. That includes fiberglass, not using dope and elecronics controls, and 2.4 Ghz. I still follow the rules as written.
Fred