Why do some Christians wear crosses? Why do some Christians wear crosses? What is the purpose of wearing the cross? What is the point of wearing the cross with a tiny Jesus on it? Is wearing a cross with or without Jesus a violation of one of the commandments of the Decalogue? How do Christians-who-wear-crosses justify their wearing crosses? 

I think this demands a cross-denominational answer, because I at least know both Baptists and Catholics wear crosses, and many priests and pastors from various denominations would wear a cross about their chests.
The Cross is a Symbol of Victory

To modern Christians, the cross is an icon, a visual symbol of victory of Jesus over sin and death. It is incorporated into countless logos and images used by Christians because it was the means by which the greatest act of love and grace was accomplished. There's a certain irony that the figure of an instrument of execution has been turned into a symbol of triumph, which makes it that much more pleasing to the Christian. 1 Corinthians 1:18-31 comes to mind in this regard:


  For the word of the cross is foolishness to those who are perishing, but to us who are being saved it is the power of God. (NASB)


People wear a cross as if to proclaim to the world that Christ was victorious over death (and therefore, so shall we be).

Idolatry?

(I'm assuming this is the commandment alluded to in the question.)

Having a picture, symbol, icon, etc. of a thing is not the same as having an object of worship. I believe that groups who use a crucifix (Jesus on the cross) use it as a vivid reminder during prayer or meditation, not an object that represents God and is worshiped as such.

I cannot provide a thorough answer for the practice of Roman Catholics, but I'm certain that they do not believe themselves to be making an idol or worshiping it. All the references I see to crucifixes, like Prayer Before a Crucifix (emphasis added), include "before" and no other preposition (like "to"). Roman Catholics and others who pray to saints do not believe themselves to be worshiping an idol or making an image fully representative of the living God.

I found a more thorough treatment of its use and history in Roman Catholicism here: The Cross and Crucifix in Liturgy.

There is no Christian practice of worshiping a cross or an image of Jesus. To a Christian, having a sculpture or pendant crucifix is no different than having a painting of illustration of Jesus. They are tools to remind us of important things, not objects of worship.