PCM is Pulse code modulation.

Lets say that you are out flying your new plane. For the example lets say you are flying on channel 40.
On that particular day there happens to be one other person at the field using channel 40 as well.

So, lets say you are in the air flying, even though you notified the other individual on channel 40, maybe later in the day he forgets.

He has made some adjustments on his trims or something and decides to test out the radio for a minute or so. Totally forgetting about the frequency he turns it on.

Now if you dont have PCM in the plane (regular FM) then your plane basically will freak out. By the time you yell out "who's on 40!!!!!" your plane is probably headed for a dirt nap.
There really is not much you can do unless the other person turns off thier radio to clear the frequency.

PCM is much different but the outcome is the same in my opinion. PCM in some cases can increase the chances of a plane to crash dramatically.
Let me go ahead and say that I have been running PCM equiped radios for years racing R/C cars. I currently run a Futaba 3PJ for my cars always in PCM failsafe mode. I would never run without it.
For planes, I dont use it.

Anyhow, what PCM allows you to do is to set pre-set settings on your servos in your plane. If a you take a radio hit like described above then the rcvr sets the servos in the settings you specified and quits taking transmission from any input source. So if I set them to go to idle, and turn into a spin then thats exactly what the plane will do. It will more than likely crash depending on what kind of interference it was.

If you get the frequency clear in time (rare) then the PCM rcvr usually takes some time to take transmissions again. IN my experience I have had to actually cut power off and turn it back on before it would recieve again. That only occurred once and I have only had a couple situations where I got glitched. All were other people flipping on radios in the pits. This was car racing of course, as I have not had any problems with my plane radio so far.

In my opinion Failsafe is more valid in areas where flying is contained in smaller areas and safety is a issue. Of course safety is always an issue but in a big open area your less likely to crash into someones home or backyard kind of thing.
Other than that I dont see the real value PCM offers. Its very costly to run and can increase the risk of losing a plane due to latency once the frequency is clear. FM will recieve as soon as the frequency is clear, if its still in the air.

Dbow