This is the first time we have anything at all disparaging about the roots of the tree. Even in this case, it is not that the roots are not good (for the Lord explicitly reinforces the goodness of the roots in verse 36) but simply that they "profit nothing."
The Abrahamic covenant is good, as it the message of the gospel delivered to all of the ancient prophets. Nevertheless, at this point in time the fruit of the tree is not "profitable." In spiritual terms, the result of having the gospel among mankind is not producing power to salvation. There may be good, there may be fruit with suitable appearance, but the final goal of the Lord o the vineyard remains unfulfilled. These forms of the gospel do not have the power to exalt.