Authors who (knowingly or unknowingly) depart from Church teaching can be spiritually dangerous, which is why the Church has an index of forbidden books, even though it is no longer updated. It is why some Bible translations were burned once upon a time, although today that would be practically impossible.
Please note that the Index still has moral force, even though the ecclesiastical penalties no longer apply (See Acta Apostolicae Sedis 58 (1966), p. 445). And although the nihil obstat (nothing forbids) and imprimatur (let it be printed) are no longer strictly required, this system is still in use and recommended by the Church (See Code of Canon Law, 827 §3).
The reason that books are no longer added to the index is not that books are no longer offensive to faith and morals, but because there are so many books, it would be practically impossible.
That it is not to say that *all *books by non-Catholics are necessarily bad, or that all books with an Imprimatur are necessarily good; there is an element of prudence and discretion here. But on matters of the faith, I’d much rather read an approved catechism, a Doctor of the Church or a Saint, or at least someone with an Imprimatur, than a non-Catholic writer.
Again, many Protestant Bibles, though missing several books and parts of two others, may otherwise be largely acceptable translations so far as they go (compare the RSV and the RSV-CE, for example), still one places oneself at the mercy of the translator; and if the translator has an heretical bias, then one subjects himself to an heretical influence.