Eh. I solo healed him without using Soil once, but using Lustrate religiously. Soil is nice when your tank has full, or near to, health. It's not a good use when your tank has < 2k health, unless you are playing the 1% game with your other healer (but then you can throw "proper" out the window, anyways). Using it proactively for the burst/spike is understandable, even if I don't necessarily agree with a specific application. Using it during the "oh crap" moments is, in my mind, not the way to use it.
Edit: I agree with your assessment of Medica II, as does my healing partner (WHM).
