I remember someone actually complaining how JMJ doesn't play the synths live in the Oxygene 4 music video. After that point you realize there's 0% content to any part of this "debate".
Of course, JMJ gets accused of other wild things too - such as not composing Oxygene or Equinoxe. In fact, if JMJ made a track with a Rembrandt violin, "fans" would be complaining about Rembrandt having composed the song. ;]
(That's the interesting difference between JMJ fans' and non-fans' attitude to this stuff. JMJ fans go all "OH NO MICHEL GNEISS COMPOOSED OXYGENS", non-fans go "does that matter, it sucks anyway" - "OH NO, HE FAKED DA SYNTH PLAYING" "who the hell wants to hear him live anyway")
But here's my honest opinion on all this: I'm really not interested in seeing a JMJ playback gig live (well, not in this form).