We all know evidence is in the eye of the beholder. That is the entire point of this - to bias the lens through which potential evidence is examined. I think we have all seen enough cases where an innocent man was convicted because police chose him as the guilty party and didn't expend any effort looking for another perpetrator - and I am not just talking about sexual assault cases.
This "believe" bias is being sought for a reason - because it will effectively bias the investigation.