Here is a definition of plain language:
"Plain English is clear, straightforward expression, using only as many words as are necessary. It is language that avoids obscurity, inflated vocabulary and convoluted sentence construction. It is not baby talk, nor is it a simplified version of the English language. Writers of plain English let their audience concentrate on the message instead of being distracted by complicated language. They make sure that their audience understands the message easily." Professor Robert Eagleson, Australia (Click here for the reference.)
This is the message that many schools promote among their students. It is also at first glance logical. After all, the purpose of writing and of communicating is to get a point, idea or argument to be understood by readers, or in the instance of other media, viewers, etc..
Clarity is a value that is celebrated in writing and journalism schools as is economy, both as to argument and with respect to communication. It is true that there is a difference between a beautifully written piece and one that seems to have no clear purpose. There is no question that writing is a craft that demands care and precision as well as an overall vision. I use the word craft here in a cautious manner largely because craft and skill seem to go together but there are many examples of skillful writing where very little is actually said.
Writing is a material practice that requires work on and with a variety of tools from pens to computers. These technologies have their own impact on style as well as clarity. But, the act of writing is more complex than the tools themselves and I would liken the process to the activity of listening both to one's inner self and to the hypothetical audiences one is addressing. The silence that accompanies writing is an integral part as well as a progenitor of creativity.
I would suggest that plain writing is a myth since it suggests a degree of control over the process of writing (there are very few effective or interesting synonyms for this word) that more often than not is both improbable and impossible.
Writing is also about reading. And, reading is neither a precise nor a particularly linear or simple activity. For the most part, the act of reading is built on inference. There is no simple relationship between a metaphor for example, and what a reader does with it. Interpretation is an integral part of reading and good writing opens up as many possible ways in which the reader can take ownership of the text. Combine interpretation with listening and you have a rather wonderful and often symphonic process that metaphorically speaking, allows words to float off the page. Readers play with all the elements, connect and disconnect the meanings and build messages, meanings, metaphors and images from the texts.
An argument could be made that plain writing demands clarity from the reader. The reader needs to shape and reshape meaning in order to make a text work for them. Reading needs to be understood as a challenging activity, one that requires work and participation and most importantly a balancing of the forces between writing and understanding.
So, if you find a text difficult demand more of the writer but also perhaps demand more of yourself. (My two cents!!!)