Bradley Manning will be a wild card in any Assange trial
He has nothing to gain by covering for Assange. By virtue of his pardon, he can provide detailed testimony regarding how WikiLeaks operates with absolutely no fear of punishment. However if he covers for Assange and is found to have perjured himself, he could face further prison time
Obama may have issued a full pardon to somebody that may end up providing testimony damaging to members of the Obama Administration