I think I remember of a study done once where corn was compared in 15" rows vs 30" rows, in a dryland condition.  As I remember, the 30" row corn did better.  As I remember, the explanation went something like this:

in 15" row corn, the corn plants have access to the full range of soil moisture at a sooner stage in developement.  The corn plant uses up all the soil moisture earlier in the vegetative cycle of the plant; when it comes time to pollinate and fill the ear, there is less moisture available in the soil, and the plant is stressed more.

In 30" rows, the corn plants - being further apart in between the rows - don't use up the soil moisture in the row middles until later in the plant's life cycle;  the plants get stresses sooner in their life cycle, but have (slightly?) more moisture available later in the season.  

I don't know if I am making this up, but I do seem to remember hearing about this somewhere - but I don't remember where........

If this is true, the principles would carry over to beans as well, I think.

Has anyone else heard of this train of thought?