It feels like most environments for editing code have line numbers hidden by default. For example, Eclipse, IntelliJ, Oracle SQL Developer, and Vim all seem to have the default setting as not showing line numbers, and so developers must go into the settings and turn line numbers on if they would like to see them.
Displaying line numbers is essential when debugging code in an environment where errors are reported by telling the developer the number of the line the error occurred on. Line numbers also give the developer a quantifiable sense of how long his code is and, in the case of large files, a sense of where in the file he is. Not to mention, line numbers are absolutely essential when doing any sort of partner programming.
So why is the default behavior in so many text editors to have line numbers off? Is it because of the aesthetics, i.e. having a cleaner look without line numbers? It feels like it wouldn't be, as applications made by developers for developers seem to be the exact sort of scenario where function would be chosen over form.