Should sci-hub take arXiv under its wing? A great way to gain legitimacy.
Is that sarcastic? arXiv is legal and mainstream, used by professionals; sci-hub is illegal.
The simple reason was that I got conditioned to doing that because the login wall to doing it legitimately was often too painful. At a certain point my workflow switched to checking scihub first, then going the legal path if scihub failed.
It saves a lot of time, and I suspect it is a lot more common than most people think.