We found a great article discussing pros of two-stage ditches, you can check it out here - http://www.balticdeal.eu/measure/two-stage-drainage-ditch/
This article shares a great, concise description of two-stage ditches, highlighting their erosion, flood control and ecological benefits, control of nutrient loss, and the fact they require less maintenance.
When used to clear out, or re-shape a ditch, a properly built Ditch Doctor Rotary Head Attachment that is specifically designed for two stage ditches can be more efficient than traditional bucket methods, in fact, since using our Ditch Doctor Rotary Head Attachment to maintain drainage systems we have increased our metres ditched per hour by 262%. I understand that seems like an unrealistically high number, but it's all about finding, and properly implementing the right tools to perform a job.
As for the increased area required along the side of the ditch to properly implement two-levels, over time this self-sustaining structure will not expand as a typical 'U' shaped ditch does. Traditional ditches tend to get wider and wider each time they are cleared out, with the proper use of a Ditch Doctor Rotary Head Attachment, this can be avoided. After all, land structures are here to stay, a quick and easy fix at the expense of long-term durability seriously harms long-run potential.