In short, calling others racists or sexists or communists etc. tends to sharpen the boundaries against free thinking that we are against. Why?
Useful information is like a virus. It spreads and affects us to the degree that we value it's utility. If the information is contradicted by experience, it loses its veracity and hence some degree of its utility. Curiously, humans do seek out and value thoughts which are often in contradiction with empirical reality when those ideas are useful to our emotional well being. This last statement is simply saying that denial is a natural stage in learning just as it is in the stages of grief.
However, unlike religious convictions that are neither provable or refutable, the utility of beliefs in contradiction to experience is transitory for any learning being that evolves towards it's own tangible benefit. Denial yields to anger to bargaining and ultimately to acceptance of the new experienced truth. That is, of course, while we allow ourselves to evolve. The ideology of the "isms" act as a learning impairment. When we label others with a derogatory group membership as "racists" etc. we are actually defining in our mind a justification against learning. What we are not doing is teaching the object of our derision. Whatever the good that we might offer it is effectively nullified by the symmetrical onus that we confer since all people want to feel good about themselves while they think about the things around them.
Learning is its own joy. To the degree that our information is useful, the new found utility of the information is a source of empowerment and so naturally appeals to all. The solution of all the pathologies of the isms is found through mutual learning--not by name calling.
So when you next hear of someone being called an "ist" ask whether one is solving a problem or contributing to it.