[quote]Native species of grains? What agriculturally useful grain is this you see growing out in the wild? Rice, wheat, and especially corn are all dependent on man to cultivate the soil and plant them.[/quote]
Rice, wheat, and corn in the form we commercially farm them are moderately dependent on man to cultivate them.
Amaranth, spelt, and others are all capable of supporting and spreading themselves.
As to their use... well, for the most part, they're of fairly limited use. But it's always useful to have a wide gene-pool: pretty much any kind of animal breeder can tell you that it's a good idea to keep a few outcrosses available in case the primary bloodline gets too inbred. The day may well come when some pest or fungus evolves that will decimate commercial modern wheat, and nothing else. It'd be nice, at that point, to be able to work with some other grain.
In addition, those are also pretty useful for people who, for whatever reason, can't eat wheat. I know three or four people who can't eat wheat -- it makes them violently ill -- but have no problems with amaranth or spelt.