Quality guru Ed Deming said that true innovations seldom come from within an organization. Therefore, perspective is probably the single most important thing that a consultant can bring to you and your organization. You shouldn't be surprised when your consultant provides insight, you should expect it from him. You pay a premium with most consultants, at least in theory, because they've been there and done that. So it's important to screen your consultant at the front end to be sure she's had experience with organizations going through the same challenges as yours.
Many organizations use consultants to do the dirty work during layoffs or cost-cuttings.
Although it's important not to abdicate your decision-making power, there can be a value in having the hired gun take some of the heat when there's a tough decision to be made. Effective consultants should be able to outline a specific plan to get you from where you are to where you need to be. A consultant can, and should, offer suggestions that aren't tainted by territorialism, history, or political alliances. The beauty of cooking dinner on your engine is that by the time you arrive at your destination, dinner is ready. Pick your consultants carefully and you won't end up as road kill.