Earlier quoted context omitted.
Sublime-text-style multiple cursor text manipulation would break here. And your code would look different than everyone else's machine. How would you display a guideline to make sure you were under a characters-per-line style? What advantages are there? I could never see it working for me. I want a program to format my code with resonable defaults and be done with it.
How would multiple cursors be affected by this? I use a proportional typeface and multiple cursors for all sorts of things, I can't really see how these concepts are even related. As you say, use automatic formatting and you don't need to worry about line-length. (Otoh, having a mandatory max line length makes about as much sense as having a max identifier length or a max lines per file.)
Line length guide lines makes sense for me as a visual indicator of what will wrap when I hit the max lines per file. They are also nice ways to gauge when you are nesting too deep or making overly complicated expressions.
Max line length allows you to do nice things with window management and diff tool layout and not spend forever moving around to see the text. I am a convert after thinking it didn't matter. Now it being 80 characters is arbitrary but so would almost any other number be.