The main reason the engine is warmed on PFL's is to detect the formation of carb ice before it is too late. If the engine has iced up you will know as soon as the engine starts to gag when the power is brought up. Therefore it is important that the engine RPM is raised to at least a cruise RPM value and held for long enough to ensure the engine will maintain its power. Obviously any sign of a reluctance to accelerate and/or maintain power should result in
terminating the exercise and positioning the aircraft for a real precautionary or forced approach.
Since fuel injected engines are much less susceptable to icing this is less important for aircraft with these engines. I still like to see the engine cleared just for my own piece of mind that the engine is still there.
Finally IMO the only acceptable way to simulate an engine failure is to slowly close the throttle.