Yes, Manning et al. attempted to do a public service and got punished for it. That seems messed up. And it may be that the information they released is important enough that we all have a right to know it. (I think this is more plausible with Snowden than Manning, but both may have a claim here.) But there really are also serious questions about whether it is OK for an individual person to make the decision for all of us about what needs to be released and what does not, even if we like the outcome in a particular case.
What is the answer here? Should we really just decide not to prosecute government and military personnel who reveal sensitive government information (which is indisputably a crime) just because we think the release was valuable? Who makes this decision? What if DOJ or the president like that the leak happened, but the people do not? Are the people well enough informed to make this decision? What about the issue of deterrence? Even if everyone agrees that a leak was worthwhile, is it really a good idea to empower others, whose judgment may not be so good, to do the same thing?