Certainly, if the case against Osama Bin Ladin was weak, we have to ask ourselves "why then was the US bombing Afghanistan if the evidence was weak?".
Here I will present a report from The Independent's online newspaper criticizing the "evidence" against Osama Bin Ladin and showing that it was baseless:

What the Government's dossier against bin Laden doesn't say and can't say: One thing is missing from the document 'proving' Bin Laden's guilt – the proof