Can't argue that, but MJ seems a bit too focused on the technical aspects of music at this point, and those of us skilled enough to appreciate those differences are often forgetful about what it was like when WE couldn't play, yet...
He's got a while to go before getting the TOTL is going to do anything more than just make life more complicated for him. S910 sounds like a VERY good starter arranger. He'll know when he really NEEDS something better to progress if he listens to himself critically..
If noodling to new age arpeggiation backing is the END point of what he wants to do, then perhaps now is a good time to get an M3 or Oasys, and bliss out on 'heart of space' music to his content. But if he wants to progress further than he is now, sticking with a good all-rounder (some of the S910's pads and synth sounds are quite excellent) and concentrating on some basic lessons will yield the better, faster improvement, I think...
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An arranger is just a tool. What matters is what you build with it..!