Circumstantial is not a synonym for weak. Often, the opposite, when you think about it, as the other kind of evidence is witness testimony, which although held in higher regard, can actually be far less reliable. Circumstantial evidence encompasses all of forensic evidence, up to and even including high definition video of the perpetrator committing the crime, walking up to the camera, cutting himself on the lens and leaving behind a full set of bloody fingerprints.
A witness on the other hand could be remembering wrong, or could not even be a witness - they could have imagined their witnessing based on information leaked to them (on accident or on purpose) during their interview.
If I'm on a jury, I want to see all the evidence, and I'm not ignoring something just because it's "circumstantial." Insufficiently specific on the other hand..., well, without the confession I'm sure that defense lawyers would have brought in experts to testify about the number of other Harvard students also using Tor.