Phytates are, essentially, how grasses and beans store phosphorous. Phosphorous is important for building strong bones and teeth, and it plays an important role in releasing energy from fat, protein, and carbohydrates during metabolism. It is also involved in the formation of genetic material, cell membranes, and many enzymes. So phosphorous is a good thing. Unfortunately, in the form of a phytate, phosphorous is not available to humans for digestion. We don't have the right digestive enzyme (phytase) to release the phosphorous from the phytate. So that's one problem.
The other problem is that the phytic acid binds to important minerals such as calcium, magnesium, iron and zinc. This can lead to some serious nutrient deficiencies in the bodies of people who eat a lot of wheat, barley, rice, rye, oats, and other grasses, and beans. (Those of you who train with me have likely gotten your magnesium lecture already, but I'll be posting more on getting enough magnesium at a later date).
To conclude: the hierarchy of beans/grains is as follows: processed/white foods at the bottom of the barrel-- keep these to a bare bones minimum-- once or twice a month or less (i.e. white bread, white rice, processed soy "meats," etc). Whole, unprocessed/minimally-processed foods are next-- choose these over white, processed products (whole beans, tofu, whole grains, etc). Sprouted/fermented items at the top-- choose these first whenever they are available (tempeh, miso, natto, sprouted beans and grains, etc).