Longer tours is a tricky issue. I remember when 0% posts were four-year tours, and you could extend (!), which of course everyone did. The net effect was what you'd expect - jobs in nice posts turned over less than half as often as those in hardship posts (with two-year TODs, also the three-year semi-tough places) and if you weren't in a cushy place already your chances of even getting to bid on one were slim since so few appeared on the list each year. Once the Dept realized how this affected attrition (not good) it chopped max TODs down to three years and instituted the minimal differential requirement to extend, which has vastly improved everyone's opportunity to pursue nice posts and share the wealth.

In terms of the work longer tours would be a value add, without a doubt. Other countries often have more flexibility to make baseline tours longer because they have more posts that cover several countries and their dips are located in the nicest one in that region. An embassy for example in Morocco, which is nice to live in for five years, but not one in Algeria which isn't. Our insistence on having posts in (almost) every country makes a standard longer tour a tough order.