The utility of D&B depends on your business. If you are a tech startup in Silicon Valley odds are you will never have a use for D&B. However, if you are a startup in the DC area planning on doing business with the federal government, you will need D&B. The federal government requires all companies doing business with it to obtain a DUNS number (an ID number provided by D&B). If you want to do business with the federal government you will have to "get in bed" with D&B - there is no way around that. Then there are those startups that fall somewhere inbetween these two extremes.
How useful is the D&B rating on the business level?
As far as the D&B credit rating, it's useless. Most entities will make their own assessment of your company's credit worthiness. On top of that, as a startup, your personal credit rating and your business credit rating are intimately connected.
We received the same letter you received (or a similar one) from D&B. It stated:
Managing your business and personal credit separately can help protect you and your family from personal liability. When you put your own credit on the line for your company, you may be personally responsible for any payments your company is not able to make...
What they don't tell you is that as a startup with no credit history this is already happening. Most creditors will require that someone (usually an owner) personally guarantee the debt. This means that if the business can't repay the debt, the creditors can come after that person for the balance. You will be "personally responsible for any payments your company is not able to make" whether you pay for a D&B credit rating or not.