Does this not over-moisturize the hair? Having the hair rest on a waterbased products (like conditioner) all night can result in weaker, albeit very soft hair.

Prepooing with oils (like olive or coconut) is good because when they penetrate, they'll protect the hair from absorbing too much water/expanding too much during the wash. Doing it with water can feel very nice but the strands can lose strength in the process.