As long as what was not said is irrelevent within the scope of what was said, isn't such an argument non-sequitur?
For instance, Obama and his wife have withheld from public scrutiny a great deal of their college records, writings and his work with Ayers. The omissions may cause a rational person to think they have something to hide. The conclusion follows from the omission.
Likewise, if the defendant does not testify in his own trial, a juror may conclude he or his attorneys believed his own testimony could be harmful to his case.
Unlike in "real life," in a trial, the judge may rule on pretrial "motions in limini" so that the jury is not even aware of evidence which he deems as prejudicial to the defendant, e.g. if the defendant had been previously convicted on charges which do not show a pattern of conduct related to the case at hand.
In a recent child molestation/murder case in California, the defendant had been in a plea negotiation for a reduced sentence in return for telling them where the child's body was located. While still negotiating, they found the body and the case was tried. That plea bargaining was withheld from the jury. Had the jury known there was an omission, even if they didn't know the substance of it, it would have influenced their decision.
Relating all these insights to Darwin's omissions - the more people know about his family, his affiliations, the reactions to his theory and his own private communications - the more justified they may feel in filling in the blanks. In their minds, it is not a "non sequitur" - but in your mind, it probably is.