Even if each piece of evidence could be considered weak or unreliable on its own, the sheer amount of it makes it reliable and strong. Matching one point of a fingerprint doesn't mean much. But matching 12 is conclusive.
In this case, each piece of evidence forms a few pixels but when you put them together they display a high resolution picture that is unmistakably Oswald.
Known formally as: consilience of evidence.
But one has to be able to consider the totality of evidence and not isolate one piece while ignoring the rest. Which is what the Oswald defenders do, consciously or not.
It's also why, for me, the alternative idea of this being planned and carried out - in secret - with some larger force directing it is simply not plausible. Too many parts, too many people, too many aspects to control or direct. Read some of the millions of pages of documents that the government has released. Other than seeing the astonishing amount of paperwork that government generates (good lord, memos and cables and cables and memos about other cables and memo ad infinitum) one can see the confusion and disarray of the people inside government. Even people like a Hoover or McCone were unable to keep on top of this.