Surely the reason you're taught to aim 1/3 of the way into the field is to allow for gusts, windshear and the like causing a sudden undershoot?
And let's be honest here, the fields most PPLs practice PFLs into are hardly likely to have any significant curlover/windshear effects, unless you're flying in some particularly sporting terrain. So it therefore makes more sense to practice the last 2-300ft on a PFL or glide approach to an airfield, where you can also prcatice judging a flare from a higher RoD and IAS than that usually experienced in a powered approach.
As a further point, I would advocate practicing PFLs to airfields as well as to fields - is this not the method of recovery you'd use in the event of, say, a rough running engine?
Just my 2p's worth!