With my B1750 I've found that in 2WD the front wheels tear up the turf while turning sharp corners. The B1750 has the bi-spead turn where in 4WD the front wheels turn faster during a tight turn, pulling the front end around and this seems to protect the grass fairly well so in my case leaving the tractor in 4WD is preferable - even on flat ground.
As I understood it the bi-speed turn thing is a way of adding more lead to the front to allow for it being on a wider arc.