I once heard a very fine lecture about the passive voice and why it gets such a bad name. The speaker said that we, as readers, expect and prefer our sentences to proceed like this:
actor > action > thing acted upon
The judge > applied > the rule.
In fact, he said we find this order easiest to understand. Note that this pattern follows the subject-verb-object order as well. But when we write a sentence in the passive voice, we change this preferred and expected order:
thing acted upon > action > [actor]
The rule > was applied > [by the judge].
This order is less than ideal; we disrupt the preferred order, or we reverse it. Even though it is still subject-verb-object, it's the actor-action order that is key for readers. That's one reason the passive voice is undesirable in professional writing: it disrupts the expected, preferred, and easiest order for our readers.
Here's another reason: Notice that the actor is in brackets. That's because, in a passive sentence, you can omit the actor. Omitting the actor can be frustrating for readers. "The rule was applied." By whom? Don't leave your reader asking that question.
Sure, sometimes the actor is irrelevant or is not the focus of the sentence. "What about the rule?" "The rule?" "Yes, the rule." "Oh, the rule was applied." Here we are talking about the rule, and it is the focus. But most of the time we are talking about actors doing things, and the passive voice allows us to omit the actor. Think carefully before you do.
The passive voice also allows you to obscure the actor by placing the actor at the end, in a prepositional phrase. "The rule was applied . . . by the judge." This frustrates readers, too, because the actor is generally the most important thing to know, and the reader must wait until the end to get that information.
And the passive voice usually requires more words:
The judge applied the rule. (5 words)
The rule was applied by the judge. (7 words)
Other things being equal, the shorter way of saying something is always better. Always.
Finally, a document full of passive-voice sentences will put your reader to sleep faster than a warm office and 106 legal memos.