It's difficult, because there are some pros and cons of doing it. People who normally share essays can be assumed to be at the top of their grade for English, and usually be able to produce A range essays (18-20). These essays mostly incorporate complex language and ideas which not everyone may be able to understand themselves. Simply regurgitating or writing an essay for the sake of it without understanding what it's about or even changing it to answer the question will bring about consequences. While you may not be caught for plagiarism in the HSC (solely due to the sheer volume of essays they have to mark; around 30,000+ I think for English Advanced), memorising an essay which doesn't actually address the question will definitely result in not being able to get the best marks you can get, and will automatically prevent you from getting those 18+ results.
That said, I don't think there's anything wrong with rather sharing ideas and main themes of texts, as these can be moulded into any question which is given.