[beevai142]

Publishers (in this subfield at least) usually demand ownership only on the final typeset manuscript PDFs. Those cannot be uploaded, but people are usually free to update the arxiv manuscript by uploading their own "final" version files, with content equivalent to the published one. In the corner where I come from, I'd say this is done most of the time, especially if there are major changes. In practice, people often read only the arxiv versions anyway since publisher's web pages can be crappy.

Also, since you submit manuscripts to most journals in TeX, there's very little extra work involved in uploading the updated files also to arxiv. You maybe miss the copy editor's grammar corrections etc., but those are almost without exception unimportant --- also, more often than not, the copyediting by the publisher introduces errors not present in the original manuscript.

[kkylin]

Agree with everything. I also want to point out that the final published version is not always better -- it represents compromises made with reviewers / editors to get papers through. Often these are positive, but not always. Sometimes it's useful to be able to send people the preprint rather than the final version.