because you need circulation with convection currents to drive the crust. if it were flat, this couldnt happen because there wouldnt be anything making the core spin, therefore no circulation. unless theres some magic making it spin. or DE, or whatever excuse you have.
That is exactly my point though - why would you need a sphere for this process? What I am saying is this:
you have a surface (crust) broken up into fragments of various size and shape. They are all slowly grinding, etc. due to your afore mentioned currents... why would this at all require specifically a spinning core? I would think that just basic physics, with any kind of hot core or layers would create massive amounts of varying pressures, and with the obvious variations in the Earth's makeup would cause all sorts of different 'pushes' and 'shoves' small and large, causing slow movements over time, venting, quaking, etc.
So, basically my point was that if we are assuming that the Earth is indeed flat, then I think the mechanics would also work out slightly different, and I don't see why it wouldn't work out just fine with some hot layers of liquid and gas.
Hopefully that made some sense anyway.
Take care,
- Optimus