Most of us put a lot trust in reviews. To Linda's point, negative reviews that overwhelm the conversation about a business will certainly turn potential customers away.
That said, *any* review activity can increase the visibility of a business. Reviews are a "sign of life" not just to search engines, but to the site that hosts the review, increasing the likelihood that a given listing will be shown in search results or "related listings" widgets.
And getting one review--no matter how bad it is--is the first step to getting many reviews.
So I would *not* be inclined to have a client of mine "lie low" and try to dodge negative reviewers. Especially where the business owner can learn from and respond to the criticism in a constructive way (not all review sites allow owner responses), any review is more valuable than no reviews at all.