Is climate change overblown? What about the positives? Your examples don't change the fact that it is logically incorrect. In the case of Manson, it shouldn't follow logically from the fact that he's untrustworthy but because we have statements from witnesses that better match the facts. If his statement would match the facts and not contradict other statements, then regardless of his trustworthiness, we would have to acquit him due to reasonable doubt. If the statements of witnesses cannot be corroborated by facts, they should not be used no matter the presumed trustworthiness of the witnesses as opposed to those of Manson. — Benkei
If you assert an epistemological standard as high as "beyond a reasonable doubt," then I suppose we'll be forced to deny the existence of all sorts of generally accepted facts. For example: What time will Benkei be in today? Well, for the past 10 years, he's shown up at 8:30 to 9:00, so I'd say 8:30 to 9:00. Fair enough, although I can't say that beyond a reasonable doubt.
That is to say, you've inserted a non-common sense legalistic principle in here. I will say pretty unequivocally, though, that if you asked me whether I believe beyond a reasonable doubt that the waters will rise in the Netherlands in the next 100 years to the point of making it uninhabitable, I'd say no. I'd also say that I couldn't assert the opposite either to that level of proof: that the waters won't rise to that point.
To be sure, though, if Manson told me anything that seemed at all incredible, I'd be justifiably less likely to believe him than if a close friend told me the same thing. Why? Well, because Manson is a known psychopath who values truth and his reputation very little. That tidbit does matter.
A juror will be struck for legal cause if he or she indicates a financial interest or a leaning or bias for or against either party. I seriously doubt you'd find it ok to have your wife preside over your divorce proceedings (not that it shall ever to come to that), for example. Per your reasoning, we should not object to any holding by the biased judge or juror because credibility of the decision maker (whether it be a judge, juror, scientist, witness, or whoever) is an irrational criterion to consider when evaluating the decision.