Actually, I was a very strong critic of the fact that all the BK series until you got to the BK-9 had no internal sequencer or any way to capture MIDI unless you hooked up via USB to a computer or laptop... You could record audio, but not MIDI internally.
I am a strong believer in having a simple MIDI capture in the arranger itself. You usually get MUCH tighter timing that way than slaving a computer to the arranger's clock (or vice versa) and transferring what you play down the USB (or MIDI) pipe, especially with how much sys-ex tends to get used, these days. And yes, I agree that having a simple capture onboard is important for when inspiration strikes at a gig, or at a friend's, or just jamming at the pool!
My only take is that, once that is done, editing is MUCH faster, simpler and easier in the computer. Yes, you can do MOST of the editing capabilities of something like Cubase inside your arranger... But it takes five times as long to do anything serious to the sequence. No argument that you CAN do it all in the arranger, but why slow yourself down unnecessarily?
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An arranger is just a tool. What matters is what you build with it..!