##All lines numbers
All lines numbers
To clarify, as the question surfaced a few times, line numbers do not affect copy & paste, in other words, line numbers are not stored in the clipboard when you select the code.
##DRAWBACKS
DRAWBACKS
After some experimentation, it appears the line numbers do carry over using IE 11, but not on other browsers tested. This would make the copied code not compile until they are removed. This may be a pretty big deal breaker, as SE supports IE 11. (thanks @MarcAndre for pointing out)
Also, it would be nice if answers could retain correct line numbers when posting snippets; however there is no way to make a code block not start with line 1. Code in the question occasionally changes (i.e. it might get formatted a bit better) - answers referencing specific line numbers would potentially become confusing.