I have a bike with plastic fenders. The front one flutters quite a bit at any speed much beyond 30 MPH. But my other bikes have aluminum fenders. The "race" bike has very narrow and light fenders (mounted with very thin stays) that move a tiny bit at very high speeds (40+ MPH). The other bikes have wider aluminum fenders that don't deflect at all (at least to the naked, watering eye) at 50 MPH and beyond.
A spoked bicycle wheel is a lousy object from an aerodynamic POV. The idea that drag could be lessened by fairing it makes perfect sense to me - as long as your fairing isn't wobbling around in the breeze too!