Rather than say anything else on this I'll just let Yamaha's reply to my question (on the Motif Forum) regarding the MM6 and arranger features say it all.

Posted by Yamaha Rep at Motif Forum
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The MM6 is pretty unique. It actually uses a lot of the technology from arrangers and is built in the same factory as our portable keyboards.

But the OS, sound content, drum patterns and grooves were done by the same team that does the Motifs.

So sound wise it has the Piano from the original Motif, the acoustic guitars ( and arps) from the ES, the sweet flute, etc.
There are plnety of the cooler synth sounds , too.

Also there are grooves with full backing parts, bass and drums and just drums. No polkas though, just hip hop, r and b, electronic and pop stuff.

It's kind of a new idea, but we think people are going to like it a lot.
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Well there ya have it. The first sentence of his reply sums it up.