PPM is an analog signal, noise on the signal will get passed to the servos as position information and you get whats seen as a "glitch"

PCM is digital, the reciever checks the frame against a precomputed value, if its good its sent to the servos, if its bad its discarded and the next frame is read. You could drop several frames and never know it. If enough bad frames in a row are recieved the system goes into lockout where your servos goto failsafe, either last position they held or neutral with throttle at idle or cut. As soon as it picks up good frames you get control back.

That makes the plane/heli less dangerous in a crash and as aerowoof said, its not going to fly off into the sunset.

Disadvantage is cost and the checksum computation takes some processing cycles that would other wise be used for sending position information. The latency is small and probably most of us would never notice.