In math and theoretical physics the arxiv is the default place to publish and where one goes to find the best and most current work. Papers take a long time to review (years) particularly hard and good papers and publication in journals mainly serves the needs of bean counting dean's and quality control morons in the administration. Actual scientific communication is via the arxiv. Self selection mitigates most ills you describe someone who regularly posts crap on the arxiv will do themselves damage professionally and disappear.