The whole discussion of AM/FM PPM/PCM has been discussed here on RCU by many of us many times.

Being a semi-retired communications tech, I can assure you that PCM is the little Radio Control hobby world's way of catching up with the big boys at Motorola, etc. When Futaba (who makes commercial communications systems as well as our RC radios) gives advantages for PCM, over traditional PPM it is worth listening to.

You can go read Futaba's reasons, here are mine...

Much of the commercial and Military applications went to digital years ago. These digital systems are often much more complicated than our PCM, but work on the same principles. The real world went to digital systems for several reasons: error correction, encryption, audio noise compensation, etc. Being computerized, digital conversion was nessesary early in the process.

The primary reasons for PCM in our RC hobby were for error correction and failsafe.

Using IC's, your transmitter (AM or FM or PCM or PPM) takes the various commands from your sticks and switches and pots and converts them into a signal. This signal cycles about every 20 milliseconds or so (100's of times a second). This cycling signal is called the "Pulse Train" and can be converted into PCM if your transmitter is capable. When the pulse train is PCM, each 20 millisecond cycle is referred to as a "data packet."

Looking at the signal received by an RC receiver (with an occilliscope) that is being hit with spark ignition noise, it is easy to see just what the advatage of PCM is.

The spark interference can completely mask over your transmitter's signal, but only for a several milliseconds at a time. The overall signal is very clean, but is broken up by these "spikes" caused by the noise. With PPM systems, these spikes are sent to the decoder portion of your receiver and to the servos, resulting glitches and difficulty controlling your aircraft. The high speed servo deflections can cause increased current drain (shorter battery life) and other problems.

A PCM decoder will ignore many bad data packets as bad info and "hold last data packet" until good data packets are heard. This often happens several times a second.
In this condition, you may not be aware of any noise at all, as controls seem unaffected.

If a certain amount of time elapses (often selectable in the programming of the radio), the PCM receiver will instead go into a preprogrammed Failsafe setting. This allows you to kill throttle, possibly to level out elevator and ailerons, etc. and save the plane, or at least slow down its hard landing. Better than most PPM systems that would just result in unpredictable crashes.

I should mention that many different types of noise sources can be taken care of by pcm, not just spark noise. I use it only as an example.

Many people object to PCM for many reasons and I am sure that they will answer.