non-staggered keyboards, such as the kinesis advantage, better match the shape of the hand, sometimes even with vertical "staggering" instead of horizontal.
additionally, the thumbs are the strongest fingers but are only supposed to hit the (huge) space bar. this is the problem that alternative physical layouts like the ergodox and the kinesis advantage try to address.
also, split keyboards allow you to use wrist and arm angles that match your body instead of the keyboard.
different letter layouts like colemak (good, especially modern computers) and dvorak (also good, but designed in the pre-computer era) significantly reduce finger travel and improve on finger and hand alteration. for me personally, colemak works great to prevent strain.
On a personal note, I've found no issues bouncing back and forth between and Ergodox and regular, non-split, staggered keyboards.
[0] http://olkb.com/
The bottom row is much more symmetrically positioned, and yet has the fewest letter keys.