The importance of devotion to the Blessed Virgin Mary cannot be overstated.
Okay, that's an overstatement. The importance of devotion to Mary can be overstated. It's more likely, though, that a statement of its importance will be mistaken for an overstatement.
I think it's because devotion to Mary is, for people who don't have a devotion to her, just some facts: "The following activities are representative of the cult of the B.V.M." Those who do have a devotion to her, though, know that it's not just a devotion, but a relationship.
So let me propose this example of a statement about the importance of devotion to Mary that may strike some as an overstatement:
The Church will endure where, and only where, devotion to Mary endures.
The reason is that devotion to Mary both follows from and informs the Church's right understanding of herself. Where the devotion is absent, there the Church doesn't understand herself; and where the Church doesn't understand herself, there she is already in the process of turning into something else.
Much has been (and is being) written about problems related to liturgical reform after Vatican II. Too little attention, I think, is given to problems related to Marian devotion after Vatican II -- and most of the attention I've seen has been along the lines of complaining about "'experts'" figuratively taking Rosaries out of the hands of the laity.
Such complaints may be legitimate, but they suffer from the "Dynamic-Tension®" problem: They promise the laity can change from 97 lb. weaklings into real he-men by praying the Rosary, but don't really explain why that promise is at all credible.
And I admit that this post suffers from the same problem. Here I'm just claiming that Marian devotion is extremely important. I'm not prepared to try to explain why to those who don't already know.
This most Holy Synod ... admonishes all the sons of the Church that the cult, especially the liturgical cult, of the Blessed Virgin, be generously fostered, and the practices and exercises of piety, recommended by the magisterium of the Church toward her in the course of centuries be made of great moment, and those decrees, which have been given in the early days regarding the cult of images of Christ, the Blessed Virgin and the saints, be religiously observed.
Speaking of images, this 1547 icon by Master Oleksa came from here.