An overly biased essay can immediately decrease the author's credibility in the reader's eye. Credibility is defined as the quality of being trustworthy. If a writer fails to address the other side of a debate or issue about which she feels strongly, either by ignoring it or by dismissing it out of hand, it's less likely that a reader will give her essay --- and take on the subject at hand --- any credence.
When an essay is overly biased, its treatment of the subject matter is necessarily limited by the writer's refusal to acknowledge other points of view --- or even that other points of view exist. An overly biased essay, then, doesn't treat its topic in as much depth as an essay that strives to remain objective. The opportunity to make new discoveries and draw different conclusions other than those already identified is furthermore lost with an overt bias.
The author of an overly biased essay essentially closes the door on the possibility to debate the topic with others whose viewpoints don't necessarily mesh with his. Debate is healthy and useful; establishing a dialogue can lead people to new ideas and create harmony where there would otherwise be discord. Because an overly biased essay defends its main points only in subjective terms, there is little opportunity to start a conversation.
One of the tenets of academic integrity is striving to remain objective. Professional journalists make their living according to this rule of academic conduct. When an essay is overly biased, it does not measure up to the highest standard of academic integrity and research. This reflects poorly on the writer --- even if he has researched the topic well and included supporting information, the biased tone of the paper might make the reader question whether the information presented is trustworthy.