Hmm.How important is it really? The general push now is away from flowery writing.Clarity and precision of meaning is more important in nearly all subjects
It's important if you want a half decent grade in English Language and English Literature.
Even the "Language" papers include literary texts, comprehension and some poetry. To get decent marks, you need to be able to interpret what the writer is trying to convey. That's very difficult if you havn't read much fiction as you may easily miss hidden meanings, irony, metaphors, themes, etc.
We've had a battle with DS who simply got bored of reading fiction at primary school and hasn't read any fiction at all in secondary other than for homeworks. He hasn't even read his GCSE literature books in full/properly - he went straight into "revising" them via online you tube videos and other web resources.
Yet, give him some "true" reading and he'll read it for hours, i.e. instructions, biographies, text books, newspapers, etc. He tells us he just can't see the point of reading about things which aren't true and they just don't interest him.
Personally I blame the sanitised, dumbed down children's books he had to read at primary school.