I agree it was marketing. I do understand GaryMF's remarks as well. A polyphonic synth plays quite different from a synth that can only play one note at a time. And if you look at good players, someone that can rock a wurly/rhodes isn't necesarily a good piano player, to not speak of hammond organ. (let's leave church organs out of this, or carillions, which is a keyboard instrument that plays bells, found in many european churches/city halls)

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So.. where to draw the line? I think that taking the marketing approach is going too far. But so it simplifying it to keys, guitars, drums/percussion, vocals.

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So how many did he play? I don't really care.