Brokers - Because a broker is solely focused on your unique needs, he or she can help with comparison-shopping, honing in on the best prices for the coverage you need. They can even advise you on how to best bundle or customize your policies in ways that agents might not be able to do (either because they are restricted in their policy offerings, or simply because they lack the insight into your specific needs).
All points have merit but, like any service, unprofessional service can be punished by walking. However, point #4, “market blocking” is a particularly confounding practice in P&C (I don’t think this occurs in LIfe & Health). Market blocking is a matter which Insurance Commissioners could easily correct nationwide to the immediate benefit of the customer.