The subject of Orwell's dislike of the passive voice has been covered at Language Log extensively, so I won't cover the topic at length. Orwell himself uses the passive voice quite a bit in his own essay. He cheerfully admits that he breaks his own rules, but does not draw the logical conclusion from this.
The passive voice is one of the main shibboleths of a certain kind of composition teacher, one who has seen too many badly written papers that tend, also, to use the passive voice. I never use the passive when the active is better; nevertheless, I also try to never use the active voice when the passive might be preferable. Does the passive obscure agency? Sometimes. Sometimes it doesn't. Most people who hate the passive for that reason give examples like "It hasn't happened yet." The only problem is that this sentence happens to be in the active voice. Not all sentences that obscure agency are in the passive, and not all uses of the passive voice obscure agency, since you can add the phrase "by the agent" at the end of the sentence.
What the passive allows is for a shift in information structure. Sometimes, you don't need to put the agent first, as subject of the sentence. "This bridge was put up in 1938." Who put it up? We don't care; the point is the bridge and when it was built, not the names of the bridge-builders.