Answer: I would assume you had some kind of holes in the black plastic for the water to get down into your soil, especially since the crops did so well. But I don't understand why the plastic would dry out the soil. Usually, it helps keep already moist soil, moist.
As far as putting the soaker hose under the plastic...If you watered too generously, or too often, yes, you probably would encourage rotting and disease. However, if you buried that hose just a bit, it might help get the water down into the soil, rather than allowing the water to pool on the top of the soil right under the plastic. You would need to check relatively often for signs that the soil was getting too wet. That shouldn't be too difficult if you just lifted it up occasionally to look.
You would definitely conserve water with this technique.
Q&A Library Searching Tips