If one rejects the covenant of works it can cause all sorts of problems. You lose the legal nature to have Adam’s sin imputed to us and also Christ’s righteousness imputed to us. As Louis Berkhof states,
They who deny the covenant of works generally base their denial in part on the fact that there is no record of such a promise in the Bible. And it is perfectly true that Scripture contains no explicit promise of eternal life to Adam. But the threatened penalty clearly implies such a promise. 
R.C. Sproul rightly flushes out the implications that could follow saying:
At the heart of this question of justification and imputation is the rejection of what is called the covenant of works…In this work of fulfilling the covenant for us in our stead, theology speaks of the ‘active obedience’ of Christ…Without Christ’s active obedience to the covenant of works, there is no reason for imputation, there is no ground for justification. If we take away the covenant of works, we take away the active obedience of Jesus. If we take away the active obedience of Jesus, we take away the imputation of His righteousness to us. If we take away the imputation of Christ’s righteousness to us, we take away justification by faith alone. If we take away justification by faith alone, we take away the Gospel, and we are left in our sins.
In conclusion, the covenant of works is crucial for one’s understanding of our relationship to Adam and how we are affected by original sin. To toss that out we lose the legal nature that it brings to properly understand how Adams sin can be imputed to us. Even more then that we start to lose the basis for understanding how Christ’s work is coming to do what the first Adam failed to do and how Christ can be our federal head. While Murray has done a lot of good and taught faithfully the word of God remaining orthodox, we ought to not just assume that he merely saw the substance of a covenant but choose to not use the terms. It could be an open door for others who seek to flush this out more leading to more severe consequences.
 Berkhof, L. (1938). Systematic theology (p. 216). Grand Rapids, MI: Wm. B. Eerdmans publishing co.
 R.C. Sproul, “The Covenant of Works,” Tabletalk Magazine, October 1, 2006, http://www.ligonier.org/learn/articles/covenant-works/ (accessed December, 10, 2016).