beater bars only work when the carpet can be lifted by the machine's suction. if the carpet is glued down or very heavy beater bars lose their effect.
beater bars also keep lightweight rugs and very long shags from hugging the brushroll cylinder too tightly. this saves labor on the motor and wear on the rugs.
IMO beater bars have their place, but they really should be made of metal, and the machine and or nozzle must be substantial enough to dampen and withstand the vibration that beater bars can create.