I built my first wicking bed this past summer as part of a larger permaculture garden design for an organic preschool in Cave Creek, AZ. In our situation, the pros were considerable since water conservation is of enormous importance in the valley. We do not need the thermal insulation for the winter, nor is it of concern to build for an overflow area since the desert will wick that away in no time if there is a flood. The slope of the land allowed for a natural runoff area as it were to the rest of the garden, in the case of the 30 year flood, with considerable swale and berm footpaths, irrigated basins on the slope and french drain style catch-all along the lowest edge.
Pros in the desert allowed for a larger scale evaporative cooling effect on the roots, an upgrade from the use of ollas.
Considering the only wicking bed demos I'd viewed prior to construction were 5 gal bucket or kiddie pool sized, this wicking bed area was rather large and was built as a demonstration/learning garden for the kiddos. Conveniently, the excavation of the basin for the wicking bed also prvided our native soil for the construction of the cobb wall!
As per your question about duck pond water... I wouldn't! Unless it was in very controlled smaller doses. I will stay tuned to see the science behind a solution for that, should I be way off base in my instincts.
And I concur about the hiding place for excess stone/rock!
