UD, you are kind of missing the point. If you want to stay primarily in arranger mode, the CS allows a certain part of your performance to have the LH chores taken off your hands for a bit. But not the entire song.
Add to that, while the chords alone are recorded, what you DO with them is not. So you still have the option of kicking fills and variations wherever you want, even changing the style itself... something that no SMF, even with Markers, allows. And you can stop it and regain control over the chords at any time, and return to it easily, or even quickly record a new CS with altered chords. Again, something hard to pull off with SMF's.
It basically IS a 'Third Hand', not a complete crutch..!
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An arranger is just a tool. What matters is what you build with it..!