Why? Positive reviews are often placed by shills of one sort of another. Negative reviews tend to be more honest and I find it easier to tell when it's someone with a score to settle. I find the ratio of negative reviews to positive for a product to usually be meaningful (more than whatever the overall rating is) and generally otherwise compare the depth and detail of reviews.
Yeah, I want to see the positive ones too (especially the ones with detailed and specific information--again, those tend to be more legitimate anyway) but since you'll find positive reviews for almost any product (especially true in things like movies, books and music) you need the balance.