There are many rules to writing and as quickly as new genres are entering the literary scene, so too are the rules changing.
Here is an interesting explanation of a rule of writing and its evolution:

http://writerscircle.com/2015/03/straight-from-the-editors-mouth-the-verdict-on-ending-sentences-with-prepositions.html

I see two big issues at play: 1) the rules of writing 2) writing with genre and audience in mind

It seems to me (Yiola) the rules are changing and this is in large part because genres are so rapidly evolving and being introduced.

There is much to be said for the traditional forms and genres of literary expression. And, it is the traditional forms I believe we still teach in schools. And yet with digital literacy there are many new forms being implemented and introduced in classrooms too.

I (Yiola) see writing as an art. The crisper the writing the more vivid the message. The clearer the writing the more opportunity for complex ideas.

The big question is… how to best teach writing in the elementary classroom? I remember teaching writing in the Junior grades (4th, 5th, 6th grades) with the audience, purpose and genre in mind… Now however all 3 elements (audience, purpose and genre) have grown to include several (more) possibilities.