It comes down to time and money, in the end. It takes a lot less money to trench, than it does to plow, and the difference increases with the toughness of the soil conditions. The first time I saw a Ditch-Witch V252, with a factory-fresh blade, I didn't recognize the thing. The similar machine I'd seen used had a blade about as sharp as a baseball bat, and could barely function on glacial moraine soils. The demo machine pulled two inch poly through some dense clay and rock. Not sure how long the big DW blade would stay effective, without needing resurfacing and such.