Why phase noise is critical in communication systems
Phase noise can be detected by a phase detector.  This means that in a FM receiver there will be more audible noise when the local osc has phase noise because phase noise is demodulated in the FM detector.  Digital systems that rely on phase information will experience higher error rates.  AM systems that use envelope detection are relatively insensitive to phase noise.  If a transmitter has phase noise its spectrum can extend out more than intended spilling into adjacent channels.  Phase noise can be thought of as short term drift.  Old school LC oscillators with valves had terrible long term drift but good phase noise performance.  Synthesizers have basically non-existent long term drift if the reference osc is good.  However phase noise can be very bad.  If a VCO gets noise on its control voltage pin there will be lots of phase noise.