Power and Ground planes The Henry Ott way or split ground planes
It really depends on your design. Henry Ott says it will simplify your design in general.

Off the top of my head there are a few reasons you don't want split planes:
1) It turns your board into a dipole antenna
2) Crossing a split plane with a trace is bad because it increases inductance and complicates the paths for return currents. 



There are reasons for splitting planes, in some cases it can reduce common mode noise or noise on the analog side of the ground plane. If your isolating your analog section completely (which I would not recommend, but is sometimes necessary) then you will need split planes.