The passive voice has its place. But sentences in the form, "x is something that is y," are difficult to defend. Just write "Passive is important in good writing."
(Even worse: https://twitter.com/Graham_LRR/status/387778330054774784 )
(Even worse: https://twitter.com/Graham_LRR/status/387778330054774784 )
I have not seen it established that passive constructions - where they are appropriate - are harder to use correctly or well. I have seen it asserted countless times, usually by people who don't know their passive from a hole in the ground and always without accompanying data.