I am unsure what to think about the case; as we know, he recently has pled guilty to a subset of the charges against him, and is hoping for a plea bargain for a subset of the rest. That's all business-as-usual.
I personally see great utility in the leaks he made; despite Wikileaks having been terribly irresponsible in how the information was handled on the way to the public, those documents were handy in uncovering things that normally happen behind closed doors in government.
That said, I am unsure whether a pardon is warranted. The problem with amnesty (in its varied forms) is that they undermine whatever rules they grant relief from. Sometimes they indicate a deficiency in the rules that could be fixed if laws were better. Sometimes they capture a notion of justice that can't easily be captured in law that is nontheless important. If we can't classify this situation as deficient either the laws are underdeveloped or because of the necessary bluntness of law though, then I'd argue that amnesty would be a bad thing.
Does granting amnesty effectively nullify the ability of the government to keep secrets? I think it does, unless we can explain why the Manning case is different from the general-case that obligations of confidentiality attempt to deal with.
So where does that leave us? I think we need to understand Bradley Manning's intent, and weigh whatever he was attempting to reveal against the state interest in being able to have secrets. If it's true (as alleged) that the release was a revenge thing tied to his being bullied, little sympathy is warranted. Either way, the release was so ridiculously broad that unless he had some specific whistleblowing topics in mind and didn't have the time to sort that from everything else, things arn't looking good for how we should judge this. Either way, the release was very useful to the general public, but unless it's excusable because of good targeting, we might expect that this is the kind of whistleblowing that he should take a fall for, rather than the kind that law should excuse because of some kind of understandable social interest.
Practically, I don't know enough to judge this, but this is the framework I would apply if I had more information, and the framework I would be pleased to see courts (and political figures with the ability to grant amnesty, and members of the judiciary/prosecution who have the ability to use judicial discretion) using.