There can be value in studying these sort of writings, for example, to see other views of Christ, or other traditions of what he said and did. However, the scholar must always remember that they do present other views of Christ, which are somehow unorthodox–or the Church would have accepted them.
For example, let’s consider the Gospel of Thomas. It was written later than the canonical Gospels, probably after A.D. 100, although it does use earlier material. However, this puts it 70-100+ years after Jesus died, while the earlier Gospels were closer to the experience, and thus reflect a time when people could respond to correct errors in how Jesus was portrayed.
More to the point, it hints that only those who seek and find the hidden meanings of what Jesus said will be saved. That is, it is Gnostic: it depends on a “secret knowledge” that is available only to the elite. Did Jesus come to save all people, to will that everyone be saved? If you believe yes, you cannot believe that he would then hide that knowledge away for an elite. You cannot be an orthodox Catholic and a Gnostic.
Similarly, you cannot believe that the Gospel of Thomas is inspired and should be classed with the New Testament. Therefore, it can be a useful tool for study, but we must be careful when reading it. And, for most of the faithful, there really is no point to reading it, since what is true is already found in orthodox writings, while what is false is not helpful anyway.