I would conclude that someone like Spong or Crossan does in fact damage the faith of a lot of people, not just in getting them to invest in the heresies these modernists espouse, but also in convincing many to abandon the Christian religion entirely. The contradiction of trying to hold these anti-creedal tenets while reciting the creed each Sunday is too much for many, well, more rational people to maintain. So in that wise I do not think that their presentation under the aegis of church sponsorship is without negative consequence. And it's pretty clear that some people of firm trinitarian conviction decide to go elsewhere and cease expending their energy on a futile resistance to such heresy (since after all they have no hope of correcting the faults of the clerisy). So what do the people doing the inviting think of this? Well, if it is not obvious, then a couple of guesses may be hazarded. The first is that the inviting clerics are also heretics, but lack the nerve or privileged position to be up front about it. So they get other people who aren't risking anything to do their preaching for them.
A less plainly egregious rationale would involve a cleric whose own theological thinking is so muddled that he doesn't really understand how wrong these guys are, because he cannot or will not work through the implications. But I think perhaps a third principle is dominating this, and it is the manifestation of a lack of confidence in the faith of his actual and potential charges. Too much Tillich and his ilk has got our hypothetical rector part if not all the way to believing that modern people (by which he means intelligent, clear-thinking, reasonable people like himself, not the sort of riff-raff who go to Southern Baptist or fundangelical churches) cannot take the scriptural stories seriously. But he needs these people to keep the pledges coming, so he's will be be compromised in order not to scare them off. At the same time, though (and this cannot be said out loud) he's relying on the stalwarts to remain stalwart. In other words, he takes his orthodox parishioners for granted. Or to put it in other, more damning terms, he subconsciously thinks in terms of they being those who are and who remain faithful, dismissing the possibility that the speakers he brings in may be working to undermine that faith. At the same time he is subconsciously working on the premise that the potential unorthodox modernists among his flock are they whose "faith" must be coddled and nurtured. And beyond that, our putative rector essentially holds that genuine creedal orthodox faith and instruction is a threat to these waiverers, while he acts as if the converse were not true.
This would add up to a tacit admission that it is orthodox faith which is strong and lasting and which gives hope for the future. But that contradiction, I suspect, isn't going to be worked through.