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

Reports and photographs of Iraqi prisoner abuse by U.S. soldiers have raised concern about the treatment of detainees being held in Afghanistan.

Two prisoners who died while being held for interrogation at the US military base in Afghanistan had apparently been beaten, according to a military pathologist's report. A criminal investigation is now under way into the deaths which have both been classified as homicides.