I don't think it's "common", but people do it so they can scroll to the last line and have it appear near the top of the page. It's a matter of personal preference. Some editors allow you to scroll that far without the extra lines, many don't.
What editors don't do that? I'm only familiar with Emacs and Sublime which do. I'd hate to use an editor that didn't. Hopefully he added that functionality to Vy :)
In vim, j and k will only move the cursor between lines that exist, and the screen will follow the cursor. You can scroll the screen past the end of the file using ctrl+e and ctrl+y, but it's annoying.
Yes, I absolutely agree. I'm sure I've been guilty of it (I'd rid them if I noticed of course) but it's a pretty sloppy-looking project. Not holding my breath for it to replace vim.