Thanks for the info graph.
Personally, I just see honest reviews as a form of word of mouth. As these are the strongest of recommendations and advertising. It just makes sense to have such reviews online.
While they aren't as powerful as your typical word of mouth, due to the reader not actually knowing the person whom made the review, they are certainly stronger than forms of self-edification. Especially when the person reading the views notes, that there are multiple reviews spread out in a natural progression. eg, reviews with varying dates over time (months and years, not days) and with unique and natural phrasing.