Many sentences written in passive voice are vague and confusing. The active voice is simpler and more direct.
Active verbs express meaning more clearly than forms of the verb "be" and verbs in passive voice. Forms of "be" (be, am, is, are, was, were, being, been) are weak because they convey no action and often make sentences wordy. Verbs in passive voice lack strength because their subjects receive the action instead of doing it, which takes the emphasis from the subject of the sentence. See also how to Get rid of passive voice.
Weak-- "Be" Verb: A surge of power was responsible for the destruction of the coolant pumps.
Passive Verb: The coolant pumps were destroyed by a surge of power.
Strong-- implies action, movement, life.
Active Verb: A surge of power destroyed the coolant pumps.
However, "be" verbs do not need to be replaced when they link a subject to a noun that renames it or to an adjective that describes it. "Be" verbs are also appropriate when used as helping verbs before present participles to express ongoing action.
Examples--
Politicians are politically correct.
Derrick was driving when his wife went into labor.
Also, passive voice is appropriate in situations in which the writer wants to emphasize that receiver or in which the actor in insignificant.
(Cited from Hacker, Diana. A Writer's Reference. 3rd ed. Boston: Bedford Books of St. Martin's Press, 1995.)