Mike:

Thanks for your question. Nothing usurps the miracle that occurs at every Holy Mass. Relics do not do that. In fact, in most if not all parish churches there is a relic embedded in the Altar, usually a relic of whomever the church is named after (e.g., St. John, St. Thomas, etc.). "In all, relics remind us of the holiness of a saint and his cooperation in God's work. At the same time, relics inspire us to ask for the prayers of that saint and to beg the grace of God to live the same kind of faith-filled live" says Father Saunders in his article, Why Do We Venerate Relics?

A relic is some portion of the physical remains of a saint or the personal effects of the saint preserved for purposes of veneration as a tangible memorial. To does venerate mean? Veneration (Latin veneratio or dulia, Greek δουλεία, douleia), is the act of honoring a Saint. The three angels Michael, Gabriel, and Raphael are called "saints," even though they are not human, because the Church honors them as personages to be venerated.

Relics of a Saint are collected from the body of the saint. The candidate for sainthood will have his body exhumed from the grave and examined. It is during this time that relics are collected. In the case of John-Paul, and any other Pope, even though the cause for their sainthood has not begun or even considered, I believe will have relics collected as a matter od course.

The existence and use of relics is found in the Bible and in the practice of the Church since the 1st Century (e.g., 2 Kings 2:9-14; 13:20-21; 19:11-12). In the New Testament we see the presence and use of relics also (e.g., Acts 5:15-16; 10:11-12).

Also refer to the Catholic Encyclopedia article on relics.

This is not superstition because is was a Biblical practice in both the Old and New Testaments and has been taught by the Church since the beginning. While some people may approach relics with superstitious minds or otherwise abuse relics, the Church condemns this.

An excerpt from the Encyclopedia:

The teaching of the Catholic Church with regard to the veneration of relics is summed up in a decree of the Council of Trent (Sess. XXV), which enjoins on bishops and other pastors to instruct their flocks that "the holy bodies of holy martyrs and of others now living with Christ—which bodies were the living members of Christ and 'the temple of the Holy Ghost' (1 Corinthians 6:19) and which are by Him to be raised to eternal life and to be glorified are to be venerated by the faithful, for through these [bodies] many benefits are bestowed by God on men...

Further, the council insists that "in the invocation of saints the veneration of relics and the sacred use of images, every superstition shall be removed and all filthy lucre abolished."

There is also the apologetic article from Catholic Answers,  

I hope this explains things for you.

God Bless,
Bro. Ignatius Mary