For Solr 4.3 users, what would be the benefit of using Solr Distributed Search over Solr Cloud?
Or should all Solr deployment after 4.x just use Solr Cloud, and forget about Solr Distributed Search?
There won't be any benefit of Distributed search over solr Cloud. Solr Cloud is currently the most efficient way to deploy solr cluster. It takes care of all your instances using zookeeper and is very efficient for high availability.
Zookeeper decides which of your documents go to which instance. I have used Solr Cloud in production also and it work wonderfully for high traffic scenarios.