This is a very sensitive topic. Many people do not even like the appellation of "mentally retarded." They prefer to say, "mentally challenged" or "intellectually challenged."
With that said, there are very few negative aspects, if any at all, of incorporating mentally challenged people into society. Some may argue that the the mentally challenged will not be competent enough to labor. This might be true on some levels. However, if you think about this, this would apply to all people. We are all incompetent in certain things. More importantly, there will be many things that the mentally challenged can do well.
In terms of positive points, there are too many to name. So, I will only mention three of them. First, to integrate the mentally challenged in society would be a great benefit, because all people are different and what makes society vibrant is diversity. There is much that we can learn from all. Second, the mentally disabled will be able to provide many valuable services in the work place for society. For this reason, society will benefit.
Finally, society becomes a better place when all of humanity is embraced. We become more human when we do this. Talk to any parent with mentally challenged children. A society that can only embrace certain types of people is one that is on a path toward blindness.