Sorry, Dennis, but I think you are kind of dodging the fill issue. To be honest, no, there really aren't a lot of fills in the MoXS, at least compared to normal loops. In an arranger they outnumber the main loops 7 to 4 (in a Roland), and the Intro/Endings outnumber the loops 2-1 (four of each).
But the main issue is the timing thing. Unlike an arranger, the fills HAVE to be triggered a bar in advance, or you lose sync. Not only from having to rely on your own timing to stay in meter (lots of potential for screw-ups there), but simply because of the fact that you not only have to drop the fill in wild sync, you then have to drop back into the main loop in wild sync. Every fill, TWO chances to screw up the timing. I'm not saying it can't be done, I'm just saying it is a LOT more difficult than the arranger's system.
But you are also missing one other important point. Maybe you NEVER trigger a fill unless you need the entire bar's fill, but most of us, I dare say, use fills at different points in the bar. Hit it on the 'one', get a full fill, hit it on the 'two', get a partial fill. Hit it on the 'three' get a pickup, hit it on the four, get an even smaller pickup, or maybe just the crash on the next bar. You cannot do this with the XS's system. Ever fill starts on the 'one'. Ask for it on the two or three, it is musically wrong. And out of sync, to boot.
Yamaha need to completely rethink this system for it to make any sense as a musical tool for anything other than hiphop and techno (which is always about full loops, on the whole). What they have now is a pale shadow of how well an arranger works. Admittedly, it wouldn't take much for Yamaha to change it, but we all know how resistant to change those guys can get. Best of luck getting a 76 out of them! (at least in an arranger!).
I also think those tiny buttons are ridiculously placed and sized for the job they are doing, especially as you can't trigger them from other sources. Roland's big pads are great on the Fantom's, although placed on the wrong side, IMO. Even for WS players, having them on the same side that your best playing hand is makes no sense...
Yamaha slowly creep towards arranger and WS integration, but they have a LONG way to go before the WS is a practical arranger.
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An arranger is just a tool. What matters is what you build with it..!