Ratings business

A lot of focus is placed on companies' credit ratings, and I wonder how much the relationship of a company and its assessor has on its final rating. I have a friend who worked in Moody's and lamented that they were quickly losing market share due to their less liberal view of credit scores in comparison to S&P. And it's a bit worrying to think of how things will change if the ratings model is flawed, but I guess we can't really tell. Changes in ratings will affect the company's stock price, corporate bonds, convertibles issued, etc...but to what extent?