Citing published papers while using their pre-print versions
I'm going to answer from a (theoretical) computer science perspective. 

For historical reasons, in computer science, the "published version" of the paper is likely to be much lower quality than the arxiv preprint. Conference proceedings typically have terrible page limits. Usually 8-12 pages are allowed, but I have heard of 4-page limits. Formally, the published version is not a full paper, it is an "extended abstract". 

The standard practice in has become to cite the conference paper but to read the full version on arxiv. It's the only thing that makes sense given our publication system. I rarely bother to find the published version of papers I cite, even if they are readily available.

If it's standard practice in computer science, I struggle to believe that it is unethical in other fields. The arxiv version should be good enough and, if it isn't, it's the authors' responsibility to update it (or withdraw it).