We are supposed to cite our sources so that others can find the material we found, we can go back to it if necessary, and we know what version, image, etc. we used. Those are all good reasons for citing our genealogical sources.
Personally I cite my sources because I learn about the sources when I do it. Thinking about how the copy/image came into my possession, who created it, where it was originally filed/stored makes me a better researcher.
Citing makes me a better researcher--even if no one ever reads a thing I write or thing I've cited.