[mwsherman]

Passive voice is merely a form of speech. It’s not blanket-wrong. It serves to diminish the actor, which can be a legitimate choice.

“He was diagnosed with flu” is passive voice, and appropriate, because the diagnoser is not important. We care more about the flu than the doctor.

“Mistakes were made” is evasive and serves to obscure, and should be avoided.

[stan_rogers]

It also manages clean transitions when introducing new actors. The passive voice allows you to put the known at the beginning of a sentence and the new information nearer the end. Without the passive, each individual sentence may be objectively more felicitous, but there is a cognitive disconnection between them.

It might even be worth pointing out that the "authoritative basis" for all of this passive prohibition is an unfortunate misreading of Strunk & White. The Elements of Style merely points out some cringeworthy ways to misuse the passive, and suggests that it shouldn't be your go-to voice.

[eru]

Oh, The Elements of Style even gets lot of stuff wrong. Their examples of `passive voice' aren't even all passive.

[conistonwater]

It could also be the case that "mistakes were made" is poor, but without it having anything to do with it being in passive voice. Passive voice also doesn't necessarily "diminish the actor", consider "She was diagnosed by the doctors at X, and the diagnosis was later overturned by a senior doctor.". That's a kind of a blanket claim about passive that's just bound to be wrong when you check it on actual English sentences (I think Pullum's paper I linked to in another comment addresses this well).