The plug also effects compression. Longer plugs take up more area in the head and therefore increase compression - which can be good or bad. With the fuels we use the compression is what fires the mixture, and a higher than designed compression can cause pre-ignition or "pinging" and reduce efficiency. Tossing an F plug in "just because" is not helping anything if the engine is not designed to handle it. Using a long plug with an idle bar in some engines can also allow the piston head to strike the plug - not good for plug, piston or piston sleeve if pieces break loose.