It needs to be understood that these speeds are used by the autothrust system as a reference on the approach. Select flaps 1 (slats only) and S speed appears on the PFD speed scale and the autothrust system will provide a thrust setting to maintain S speed. However, you can fly slower than S speed without taking Flaps 2 but you'll have to use selected speed to do so with autothrust. As mentioned earlier, the aircraft is not going to fall out of the sky just because you fly slower than these speeds but the margin over the stall speed in that configuration is reduced. They are useful reference speeds to use in configuring the aircraft for the approach and this is what Airbus and most operators teach their pilots. Similarly, on takeoff or go-around they are used as a reference for the clean-up - at F speed select Flaps 1 and at S speed select Flaps Zero.