ORIGINAL: Mikes machines
The advantage to PCM is the signal is much less prone to interference. The signal sent from the TX is encoded differently than non PCM. The rx is more selective and rejects more bad signals than a normal signal.
That's not really true both pulse code modulation and frequencey code modulation are susectible to the same interference, with fail safe off you'll have basically the same amount of interference either way. Then the only difference is that a PCM reciever can cost three times the amount as a normal FM. Not a big deal if you have one helo but if you have multiple recievers the cost adds up.