If I'm in planning mode and I don't know where I should go eat burgers on Friday, I don't want to know where _not_ to go, I want to know where I should go.
Of course, if I want to look a specific place or product, pos and neg reviews are useful.
But if you're in a "I'm moving across the country" mode, a negative review of a moving company may be worth much more than a positive review. (Possibly $2,000 more, in this case.)
Of course, if I want to look a specific place or product, pos and neg reviews are useful.