There are many things Paul said that were directed to a specific audience. He was phrasing his words so as to emphasize those portions of the gospel that those particular members of the church needed to hear for their circumstances and their maturity in the gospel. There are things which Paul said to a particular audience that do not necessarily apply to us today. However, there are most definitely aspects of the gospel which are enduring truths. Among these are the truths concerning the relationship of grace and works. In Hebrews, Paul was talking to those members of the church who, being Jewish, were steeped in the Mosaic law and considered that salvation came through the adherence to specified rules (or works)... for example... the law of circumcision. They needed to have emphasized to them the importance of grace.
James on the other hand, in a general epistle to everyone, felt it necessary to emphasize the importance of works... saying in James 2:17-20: "
17 Even so faith, if it hath not works, is dead, being alone.
18 Yea, a man may say, Thou hast faith, and I have works: shew me thy faith without thy works, and I will shew thee my faith by my works.
19 Thou believest that there is one God; thou doest well: the devils also believe, and tremble.
20 But wilt thou know, O vain man, that faith without works is dead?
The point is that there are works of the law which are applicable to everyone at all times. The works of the law such as loving God, loving one's neighbor, repenting, being baptized and living righteous lives to the end of one's days are necessary before the grace of Jesus will (or can be) applied to save people from (not in) their sins.
So, What Mormons are defending is the concept of the relationship of grace and works together in a covenant relationship with God... that both grace and works are necessary. Works does play a role in our salvation, but... It is the grace of The Messiah, Jesus Christ, His atonement that "justifies" us. But the atonement is not applied to the individual; that is, justification will not be applied, unless and until he demonstrates his desire (through his obedience and his works) to be saved. One may have the desire to be saved, but it will only occur for those who are obedient to the prescribed path ordained of God.