Do I really have to cite an arXiv paper?
A large number of seminal works have never been published. The greatest mathematics paper of our lifetimes remains unpublished. Not every paper on the arXiv warrants a bibliographic entry, but many do. The idea that unpublished status would categorically exclude the responsibility of citation is a bit preposterous. It puts far too much faith in the deeply flawed fraternity of conference organizers and the overworked cohort of peer reviewers, roughly 30% of whom typically fail to even comprehend the basic outline of the paper. If similar work comes to our attention during a proper literature review, we ought to cite it. If we knowingly build on someone else’s work we should cite it. If someone shares a non-obvious idea with us that develops into a paper, we should find some way to credit them. If someone writes a theory down on a napkin shortly before dying and it turns out to open a new subfield of machine learning to scientific inquiry, we should convert the napkin to a pdf, upload it to arXiv, and then cite it. We should not have to cite nonsense.
via http://approximatelycorrect.com/2017/08/01/do-i-have-to-cite-arxiv-paper/