There are secrets and then there are secrets
On the other hand, the classification system in the United States is broken. Information which would give enemies an upper hand should be kept private, but some of the things Manning released--an Army document calling Wikileaks a security threat, for example--seem to have been classified solely for the purpose of saving the Army embarrassment. The video he released could be seen the same way (though I disagree with the editorial comments Wikileaks embedded before releasing it). After all, the Pentagon has released numerous videos of air-to-ground combat, so they can't argue such video should be classified because it gives away tactics or aircraft armament. It's likely they have kept this secret--and keep many other such videos secret--solely to keep the US from looking bad, which I believe is an inappropriate use of classification in a democracy.
The classification system needs to be reviewed.