Don't believe me? Well, let's take my previous paragraph as an unwitting example: "...because it means the decision was made anonymously." Any volunteers wanna find a non-awkward way to scrub that passive construction out of the sentence?
(There isn't a way to revise what's there and get an active construction; you'll have to start entirely from scratch and say something like "...because it mean whoever made the decision is going to remain anonymous"--not terrible, I guess, but not exactly what I was saying before, either.)
But passive voice isn't only for when the subject is unknown. I love passive voice because it gives me a lot of power over the flow of information. Check it out:
Active: Usain Bolt broke the world record.
Passive: The world record was broken by Usain Bolt.
Okay, yes--the active version is shorter and more direct. That should definitely be your default setting. But consider this context:
After getting a bronze medal in Osaka, Asafa Powell swore he would break the world record for the 100-meter dash. And he did: in September 2007, he set a new world record of 9.74 seconds. Nine months later, that record was broken by Usain Bolt.This is nice because, stylistically, it's generally best to give old information before new information. In this paragraph, our focus shifts from Asafa Powell to the world record and then from the world record to Usain Bolt. The passive construction helps with this flow.
Another case where passive voice is handy is when you want to really emphasize the subject. That seems counterintuitive because the subject is often omitted in the passive, but there's this thing called end weight that give the most oomph to the thing that comes last.
(There's another unwitting example: "...the subject is often omitted in the passive...." I suppose I could have said, "The passive often omits the passive," but, again, look at the flow of information: case-->subject, subject-->passive.)
This technique is used a lot in humor, as in, "I can't believe it--I'm getting beat by a rug!" For more examples, just Google this phase (and include the quotation marks): "by a freaking"
The point is, the passive voice does have a place. Yes, it's often used unnecessarily, unwisely, and unwell. Yes, checking your prose for passive constructions will reveal many places you can tighten things up. But that doesn't mean passives don't have their place. As with most grammar and style advice, the real rule isn't "Thou shalt not" but "Stop and think."