hey, i am no expert on this but i can add some insight, ideally you hit the nail on the head, the farther around you can go the better, but there are several other considerations...
efficiency obviously, about 65% with pipe and flood and 95% with sprinkler and drops (source ksu agronomy) (oznet.ksu.edu), pumping a third less water with energy prices (and a low aquifer) is a big help financially.
You can grow a much better crop under a pivot, more bushels per water unit in my area, i am sure it works to be more bushels on less water in our area as well, however water is our limiting factor. In many cases it is significant, the difference between 150 bushels and 220 bushels on 300 gpm. fewer acres, true but in many cases more bushels
labor is a huge one, pipe is a pain in the a$$ frankly, makin sure it is running all the way through, packing rows, laying pipe to prewater, picking up pipe to plant, chaging water 2+ times a day. sprinklers you set a stop and leave, something goes wrong the pivot calls your phone.
all of the reasons above i guess, and in a lot of cases it takes as much pipe to water an odd shaped field of 50 acres than it does a quarter, lots of short runs with equipment etc. in most cases i think the standard is no less that about 120 degrees with a standard length pivot.
i also often wonder why linears are not more popular, i hear they are significantly more in cost than a center but they make alot of sense. i do see some corner systems (taildraggers) see website, a picture is worth a thousand words for this one (http://zimmatic.com/zim_agsysystem_MAXfield.asp
), they are neat, i am sure pricey to, however accomplish the same thing as a linear.
hope this sheds a little light on the situation...some one can probably shed more light on it..
Edit: Fixed the URL -Lance