Is Gnosticism a dangerous error?

Mark Goodacre has pointed out a review on two books on the Gospel of Judas (Ehrman and Wright) written by Robin Griffith-Jones. What surprised me about the otherwise historically-sensitive and critical review was the last paragraph where Griffith-Jones writes,

Gnosticism is a dangerous error and must be fought — on that Dr Wright and I are at one. But such a campaign must surely do more than satisfy the hawks at home; and, regretfully, I am not sure that with this book Dr Wright will win over the hearts and minds of those he hopes to liberate.

Why is Gnosticism considered a dangerous error that must be fought? This is certainly not the position of a historian of religion, but appears to represent the sentiments of some theologians today. But even from the position of contemporary theology I have a difficult time understanding why Gnosticism is so threatening. In fact, I think that Christianity today might benefit from listening again to the Gnostic voices which have so much to say about spirituality.