This is a great question, and one that I also have. I think the responders (so far) are missing the crucial point that you suggest publishing in both arxiv and

This is a great question, and one that I also have. I think the responders (so far) are missing the crucial point that you suggest publishing in both arxiv and the journal of choice. So the journal of choice, behind a paywall, gives you the prestige and tenure. Meanwhile, arxiv hands the information over to other academics.