Downsides?
$$$$$ Causes more problems than it solves--it will jackknife going uphill!!!
Downsides?
Possibility of jack-knifing with the Land Rover design yes but not Volvo's. The Volvo trailer has jack-knifing "stops" to prevent this.
Pictures and film of my friends and mine. http://gallery.me.com/teddsoost#100383
As far as increased gas mileage the PTO is only engaged when chosen and only possible in 4 low.