My kids have had to follow formatting requirements that specify "pages" and "margins" for documents that will never be printed. Ironically, these come from an organization called "Modern Language Association."
The Modern Language Association (http://www.mla.org) is the main scholarly society for the study of literature in modern languages (including English) in North America. "MLA Style" is one of several styles used in scholarly publishing in the humanities, and its citation format is often taught in schools.
Old editions of the manual did talk about margins and pages; newer editions make far fewer assumptions about your medium (though it assumes you are going to be often -- though not exclusively -- citing physical books and media in your work).
It's been years since a publisher gave me a "page count" in their guidelines; word count is nearly universal. I stopped assigning page counts to students around the time that the publishers stopped (a long time ago at this point), and I think most of my colleagues did the same. So I'm not sure what kerning and em-size wizardry really does for you, unless your professor is pretty old school (and hasn't bought a new edition of the MLA Handbook since the Clinton administration).
Yes, for now. I don't think essay-reading software is up to speed. ;-)
My preference would be for my reader to display every document in my own preferred format. This would be the case if essays were submitted in plain text format, for instance. In that case, the only formatting requirement would be something to delimit paragraphs.
That was the dream of the original HTML. It wasn't supposed to be a graphics rendering language. Your browser was supposed to format a document according to your preferences, or even read it to you if you're blind.
What's really happened is that teachers have dropped paper-based format requirements such as inches of margin, as they get more comfortable with computers. Today, my kids use Google Docs almost exclusively.
So maybe students should be required to submit their essays in Markdown, with some restrictions on formatting so they don't go crazy with bulleted lists and so on.
Then ideally, you'd accept plaintext submissions, and have your reading/marking software format them in whichever way you want for optimal viewing (possibly depending on your taste, device, time of day, phase of moon, etc).
That's why math journals require LaTeX - that takes the formatting steps like margins, fonts, etc. out of the writer's hands.
