There are instances when findings and recommendations by scientists have been ignored, which resulted in consequences that could have been prevented or dampened if words of scientists weren't ignored (2).
Everyone has bias towards an issue and scientists are no exception. However, because scientists are the ones who have researched the topic, they are more qualified than not to be able to advocate for the said proposal. As long as the facts presented are kept in check by a group of unrelated scientists, the issue of bias should be avoided, thus scientists basically serve as extra policy makers who have expertise in the field. If they give unbias facts and separate the facts from their opinions and recommendations, their credibility should remain intact as they aren't forcing their views onto others (3).
2). Karr, J. R. (2006). When Government Ignores Science, Scientists Should Speak Up. BioScience, 56(4), 287-288.
3). Rykiel, E. J., Jr. 2001. Scientific objectivity, value systems, and policy-making. BioScience 51:433–436.