I have a split keyboard, and I think the real benefit is not that I can rotate my wrists, but that it allows me to pull my shoulder blades closer together. I sit more upright and have less back pain.
This. I also enjoy being able to move the two halves around through the day, shifting the muscular load around. It makes it easy to accommodate different seating, standing and lying positions.
(I never figured out, despite 'ergonomics training', how I was supposed to hold my extended hands and arms upright for hours per day without shoulder and neck pain.)
