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 of it by grammar scolds. The first three points are valid and important, I think; the fourth strikes me as being irrelevant at best.
The crucial element embedded in the first three defenses, however, 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 unconsciously and repeatedly in situations where doing so does nothing to supply a desirable emphasis or to promote structural variety. The passive-voice sentence "The investigation was opened on Thursday by the FBI's Washington Field Office, she said," 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 two words 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 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." Certainly one could argue that Reagan simply 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 passive voice: "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 "stops" frivolous proposals, the receivers of proposals "prioritize" them, and the makers of proposals "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: "Allocating the cost of proposals to the business side of the house 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 the price of spelling out who is doing what in a complex sentence, rather than leaving the task to the reader.
Finally, actorless passive voice crops up very frequently 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.