Now, there exists a group of people, of which Assange is one (as far as I can tell), who believe that spying is wrong, and governmental secrecy is wrong. Thus this particular Wikileaks release, and, afaict, a lot of the Wikileaks effort has gone into promulgating that ideological goal. I would note that governmental transparency is pursued by multiple political groups across the US spectrum: police transparency, foreign policy transparency, purchasing transparency, etc. It's not a simple bijective mapping between the thought-movement Assange is with and the goal of governmental transparency; there are profound nuances and deep variances of goal within the broader 'transparency' ideal.
As for me, I am perfectly content to have the spy game played against heads of state; especially if it keeps down total system instability and human suffering.