The phrase "inestimable importance" means that something is extremely valuable or significant, but it is difficult or impossible to measure or determine its exact value or importance. Full definition
This dual narrative ofthe one institution is of inestimable importance in showing the complementarity (and not the opposition) between the two ends of matrimony as taught by the Church: «the twofold end of marriage: the good of the spouses and the transmission of life» [8]. (faith.org.uk)
Rather, it is by insisting that while the scientific approach is of inestimable importance in areas where it is relevant, it is not omnicompetent nor inclusive of every area of experience. (religion-online.org)
In short, Glenn Loury is saying that there has to be a better way and, even though he is short on details, the sheer saying of that is of inestimable importance for blacks and whites who know that we must live together and, please God, can maybe flourish together in one American society. (firstthings.com)