To write well is to clearly communicate your thoughts to someone else. Unless you are writing poetry, in your diary, or some other creative type of writing – your goal should and probably is so that others read and understand it. The passive voice confuses traditional object/subject classifications. It can cause readers to become unsure as to who is acting, and who is affected. This prevents you from achieving the clearest form of communication – which is usually the goal!
Or, to paraphrase Stephen King: With a hammer he killed Frank = bad. He killed Frank with a hammer = better.