Because if you simply define "useful" as something that your characters can put to good use, and a "useful" blue is roughly equal to a "useful" rare and a "useful" unique, then there is no difference in the items since you will find an equal amount of "useful" items of each type, and the ones you use will be random based on your own personal luck.
I think "useful" is more in the broad sense of "someone would use this" category.