I absolutely love how the previous posters have given valid reasons for choosing to use the passive voice in writing. Let me emphasize "choice." Passive voice in most writing weakens the writing and should be avoided unless it is conciously chosen for effect.
This is a wonderful question. You are correct that there are some situations in English in which the driver's action would be emphasized in the sentence. If a driver purposely hit a child with a car, most people would not use the passive voice. People would say, "That driver hit that child." However, if the speaker has no idea what the intentions of the driver were, the speaker usually focuses on the person s/he does know about. In this context, the passive voice would be far more common. In, for instance, the child's hospital room, when the driver is not present, most of us would expect to hear, "This child was hit by a car." It would sound strange to us if someone said, "A driver hit this child with a car." Using the passive voice in this case is not an act of hiding guilt; it is merely an act of focusing on the person who is present and important in the mind of the speaker. (I'm guessing that Farsi speakers would not talk about the car's driver in this situation, either. Maybe they'd say something like, "This child is badly hurt." This is, of course, mere speculation on my part.)
That said, you are correct that writers do sometimes hide behind the passive voice. A classic example of this is the sentence, "Mistakes were made." In his Political Dictionary, the writer William Safire calls this sentence a "passive-evasive way of acknowledging error while distancing the speaker from responsibility for it." In other words, this sentence is usually used to talk about problems without taking any blame. Most Americans recognize the evasiveness of this kind of speech and get very impatient when we hear it.
This is a tricky question. It's tough because there is a widespread belief that the passive voice really is a shortcut to good writing. If everything is done by something else, it removes an element of activity in the writer, increases wordiness, and removes the active voice of a narrative. There are some moments when the passive voice can be used. When the receiver of the action is more important than the doer, then the passive voice can be employed. The example from the link below is a good indication: "The child was hit by the car." Compare the active voice version: "The car hit the child." I think that there is some rhetorical difference between the two statements. The first statement is one of victimization and the passive voice helps to bring this idea out more in force. The second statement makes the car more prominent. From a stylistic point of view, it seems to me that the child should be acknowledged and recognized more than the car. The content of the question makes it evident that the child was not acknowledged in full terms, so the style of the writer shoud make it so. In this light, this means that the passive voice might be more useful for the writing.
Thank you so much for your answer on my question,but is not your answer also a bit tricky??I wonder what is your opinion about the "car driver",I mean the one who is hidden somewhere behind the passive structure of the text and the writer refuses to give any information about him to the reader...and may be he does it purposefully, as by only emphasizing on victim and highlightening on victimization(though he seems honest enough on doing this) deprives us from knowing more about the "doer"and his role on happening the hitting accident or even he reduces the importance of knowing about the one who is RESPONSIBLE.This is the point that I am interested to know,where the writer hides himself behind the grammatical availability of a language,I say a language,as some languages do not possess such capacity,as Farsi ,for instance does not make use of this structure (passive voice) a lot.I think,it has some social-cultural based supporting as well,since,conservativeness is an important feature in English spoken communities, at least it seems so to us ,as Asian people.