It shows that people are complicated and multifaceted. While we have a tendency to either laud or vilify figures (with nothing in between), in reality, historical figures can be both "good" and "bad" in different aspects of their lives... just like the rest of us.
My favorite example of this is Helen Keller[1]. Almost nobody these days knows anything about Helen Keller beyond the age of 18 - they know that learned to communicate despite being deaf and blind, but the nearly 70 years of her life after that are almost buried in history.
Which is a shame, because Helen Keller would have been a notable figure in her own right even if it weren't for her disability[2].
But because she was a member of the official Socialist party, we can't use her story as an inspirational lesson and mention that she held political beliefs that we may or may not agree with. Instead have to make a judgement call as to whether her "good" or her "bad[3]" side is more important to teach, and then forget about the rest.
Worse, this principle is applied in reverse to dehumanize people whom history has classified as villains. The result is that our history is filled with monochromatic caricatures, and we forget that, were we to see the complete picture, our modern-day "villains" may not look so different (for better or for worse!)
[1] I believe Loewen talks about this in his excellent book Lies My Teacher Told Me (http://en.wikipedia.org/wiki/Lies_My_Teacher_Told_Me)
[2] http://en.wikipedia.org/wiki/Helen_Keller#Political_activiti...
[2] Naturally, this presumes that you either disagree with socialist politics or think that history lessons should be politically neutral, which is the predominant belief in the USA.