Agree with most of the above comments, but will add my 2¢.  Drilling with SI belt meters are as accureate as a planter meter/plate. And once set, don't have to worry about seed size changeing from variety to variety.

When the sprayer guy (you or co-op) are running down (runing the tire) on the 15 in row, taking it out, then it's nicer having 7½ in rows.  Now a guy has to spray several times - for weeds and bugs, so there seems to be lots of sprayer disease in narrow row beans (20-15 in).