Dive summary:

• It's common for contractors to require that subs add them as additional insureds in  order to get more protection in case of accidents, but contract language is not foolproof, three attorneys from Dickstein Shapiro LLP warn.
• They advise that a contractor's risk manager get a copy of the actual policy to review rather than relying solely on a certificate of insurance from the subcontractor because the policy language is what governs if anything happens.
• Another consideration for contractors, the lawyers note, is that the liability limits of policies don't increase because more companies are covered. Protection that sounds good might not be much help if the subcontractor uses it all up.

From the article:

Many companies involved in commercial property construction and development are additional insureds under other entities’ policies, and thereby hold valuable protection from potential losses. ...