Variable brining is useful for severaly undersized systems. Commercial applications with single tanks is the most common or residential with very hard water and a small system. There are certain issues that arise with variable brining. Watts has a modified 7000 valve that has worked fairly well (better than most) for variable brining. If a system is sized correctly, it should regenerate weekly or less. At that efficiency, variable brining does not come into play.
Second backwash... the primary advantage is it eliminates the need for upflow brining and the unique problems upflow brining creates. Second backwash is especially useful when very low salt settings are used, it gives the end user a higher quality of softened water with only a small amount of backwash water. Second backwash is standard on every 5600 electromechanical valve, it was originally considered a design flaw, now it is a "feature".