What are the positive effects of the transatlantic slave trade?

It is hard to
imagine how someone could say that there were any positive effects of the Atlantic slave trade. 
This was one of the most evil things that one group of people has ever done to another.  I
suppose there are things that came out of it that were not bad, but they could have been
accomplished in other ways.  Let us look at two such effects.

One positive
effect of the Atlantic slave trade might be that people in the Americas became much richer than
they might otherwise have been.  It is possible to argue that the United States would not have
become the wealthy and powerful country that it is if it had not been built on a foundation of
slavery.  You could say that this is good, but you might also note that this is like saying that
it is good when someone robs a bank because that person and their family become
wealthier.

Another positive effect of the slave trade, from a certain
perspective, is that the United States ended up with a more diverse society than it otherwise
would have had.  African Americans have contributed a great deal to American culture.  For
example, black music has influenced American music so much that the two are essentially
indistinguishable.  Because of the slave trade, we are able to live in a country that is more
racially diverse than it would have been and we get to reap the benefits of that diversity. 
However, this same benefit could have been gained from voluntary migration and there was no need
to commit a horrific crime against humanity to accomplish it.

In short, I
would argue that you have to really stretch the definition of positive effects to argue that
the Atlantic slave trade had any such effects.  The slave trade is a terrible stain on our
history, one which should never have happened.