Tapwater contains carbonates of calcium and often magnesium as well as other dissolved solids (minerals) that affect both the pH and salinity of the soil solution. Watering copiously from the top so water flushes through the soil allows dissolution of the salts and mineral deposits which then exit the pot through the drain hole such that a stable pH and TDS level can be enjoyed for an indefinite period.
In closed systems, like you propose, all salts and dissolved minerals from tap-water and fertilizer/ fertilizer solutions remain in the grow medium to accumulate. This causes a rise in pH, making many nutrients unavailable for uptake; causes a high EC/TDS level, which makes water and nutrient uptake increasingly difficult as levels increase; and, contributes to skewing of the nutrient ratios in the soil solution which leads to antagonistic deficiencies and synergies that interfere with normal uptake of nutrients. Example: If someone is using a "bloom booster" type fertilizer with a high level of phosphorous, there is no way to flush the excess phosphorous from the grow medium. As the level of phosphorous builds, it limits uptake of calcium, potassium, copper, zinc, and especially iron; and, it synergistically increases the uptake of magnesium at the additional expense of calcium.
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