Many, Many years ago (probably around 15-20 years), a farmer near Galesburg, Illinois (North-Western part of the state) got some outstanding yields with wheat in 15 inch rows, prompting a run on 15 inch grain drills for a while. 

I never did hear any more about good yields from 15 inch rows after that, I suppose it depends a lot on the year, and that one year was perfect for 15 inch row wheat.

I had a 15 inch row IH drill set up for soybeans, but also had a 7.5 inch row drill.  I just never did get the nerve to try the wider rows, we sometimes have enough problems with weed control anyway in a wet year.  I always figured the narrower rows of wheat would shade easier, as in most years we didn't have much weed problems, except for garlic.

So, it is possible to get good yields in 15 inch rows, but it does not seem to be consistant enough for a lot of farmers to convert to that mode of planting.