All of us as humans have probably "used" others in one way or another at some point in our lives. There is nothing wrong with a mutually beneficial, equal arrangement in which both parties understand from the start what they are each giving and getting.
What causes the most negative fallouts are those people who, despite being bigoted, reach across community lines to feign goodwill, not because they want to overcome their prejudices or genuinely want to get to know a people or an individual despite their own bias, but because those they secretly disdain are useful to them at that point in time. In the uglier cases, such people cultivate trust with open smiles wile holding daggers in their hearts. Once those they deceived have outlived their usefulness, these users abandon their friends. Some even turn on their allies in a flash and display their shocking bigotry in its full glory.
In less severe, more commonplace manifestations of this "user" mentality", the "user" cultivates friendships with someone from an "out" group as backup company, someone to fill in the social time while more desirable company from his/her own "in" group is not available. Initiating this manner of insincere 'friendship' based on need and insecurity rather than on a genuine appreciation for the other individual will ultimately prove to be a waste of time. At the very least, the user is wasting the time of the person being used, who thought that s/he had a real friend. But more often than not, the user is also setting for the stage for future resentment that results from the inevitable sense of betrayal when one party realizes that s/he is nothing more than a dispensable tool or replaceable prop.