If they're presenting false evidence and are otherwise indistinguishable from truth-tellers, then I would guess that agreement would fall a lot or cease to happen; if they're the equivalent of random noise, then I'm not sure what would happen, but probably bad stuff if we go by Hanson's paper on communicating rare evidence; and if they're merely being selective about evidence, you can still infer stuff from their reports (the Bullock thesis in my backfire effect page would be relevant here).