Most defenses of passive voice focus on (1) thoughtful use of it to emphasize the most important aspect of a particular statement; (2) thoughtful use of it to vary the form of sentences in a piece of writing, to avoid a protracted series of sentences that share the same subject-verb-object order; (3) historical use of passive voice by excellent writers; (4) the recentness and presumed baselessness of criticism that grammar snipes have leveled against it. The first three points are valid and important, I think; the fourth strikes me as being irrelevant at best.
The crucial common element embedded in the first three defenses is the author's conscious and well-conceived decision to use passive voice. In my experience, such intentionality is rare. More often, an author falls into passive voice unwittingly and repeatedly in situations where doing so does nothing to supply a desirable emphasis or to promote structural variety. The sentence,
The investigation was opened on Thursday by the FBI's Washington Field Office, she said.
for example, doesn't have any advantage that I can detect over the active-voice sentence,
The FBI's Washington Field Office opened the investigation on Thursday, she said.
The latter is a bit shorter than the former, and avoids relegating the actor in the sentence (the FBI's Washington Field Office) to a participial phrase; the result (to my ear) sounds crisper and cleaner.
But this is all a matter of taste, I suppose, since the sentence does eventually identify the actor and attribute the action to that actor. The worst fault of passive voice is that all too often it serves to deliver action without an actor. The classic example of this fault is Ronald Reagan's famous pronouncement in the midst of the Iran/Contra scandal:
Mistakes were made.
One could argue that Reagan chose this wording because he wanted to emphasize the politically fraught concession implied by the word "mistakes"; but the formulation also has the convenient characteristic of failing to identify a source of the mistakes: The sentence identifies a result and an action, but no actor (in the non–Ronald Reagan sense of the word).
Though Reagan's formulation surely represents a thoughtful (and tactical) use of passive voice, many instances of actorless sentences do not. Consider this extended exercise in passivity:
When the cost of proposals is born by the business side of the house, frivolous proposals are stopped, proposals are better prioritized, and what is proposed is more likely to have a true ROI to the business, reducing waste and abandoned projects.
The first passive-voice element ("is born") has an identified actor ("the business side of the house"), but the next three ("are stopped," "are prioritized," and "is proposed") do not. A reader slogging through this sentence must either struggle to identify the unnamed actors (the allocation of cost to the business side "stops" frivolous proposals, the receivers of proposals [presumably managers] "prioritize" them, and the makers of proposals [presumably lower-level staffers] "propose" them) or—as is much more likely—skate over the surface of the sentence without really comprehending it. The following reformulation of the sentence is far likelier to make sense to a reader:
Requiring the business side of the house to bear the cost of proposals discourages staffers from submitting frivolous proposals, encourages managers to give priority to the most promising suggestions, and increases the likelihood that proposals will offer a legitimate return on investment, thereby reducing waste and lowering the incidence of abandoned projects.
The revised sentence is significantly longer than the original, but that's a price I'm willing to pay if it yields a sentence that identifies who is doing what, rather than leaving that task to each reader.
Finally, actorless passive voice often crops up in situations where the unnamed actor responsible for the action in a sentence is in fact the author. In these instances, obscuring the author as the source of the action promotes a sense of the objective truth of the assertion. Thus, the wording
The makers of Battery Doctor/Battery Upgrade could not be contacted.
frames a reporter's inability to reach a company while composing his story as the objective impossibility that anyone could have reached them: The company simply "could not be contacted." Again, such strategic use of passive voice may serve an author's purposes; but from a reader's perspective, it clouds and (perhaps) misleads rather than clarifying.