The idea that DI water isn't good for you is based on the belief that you're body gets essential minerals etc from our water, this is not true
"The human body gains the minerals necessary to good health primarily through eating foods, not through drinking water. The body may absorb or use the minerals in water but, in most cases, the amount would not be significant. In order for a person to obtain sufficient minerals from water, it would be necessary to drink many gallons daily. In general, neither a water with a high mineral content, nor a fully softened water, could be considered a significant source of minerals. In contrast, one glass of milk provides the mineral equivalent of multiple gallons of ordinary well water.
Note: Certain trace elements, such as fluoride, iodine, etc., may be obtained from water. However, these would not be removed through common household water softening."
That's a quote from the North American Water Quality Association.
http://www.wqa.org/sitelogic.cfm?ID=360
Doug