Dumb thread, but Imma make sense of it anyway!
...Because I'm bored.
Anyways.
First off, It's remarked that retards often have really friendly social skills.
I've known a few people who were mentally disabled, and almost all of them were irritating to the core, or just plain asshats. It's a myth, just like "rich people are never happy". It's there to make us feel better about them in some way.
Truth is that yeu can be just as much of a jerk by being a 'retard' as yeu can by not being one. Handicapped people have some individuals blindly cater to them and defend them from everything no matter how deserving of an attack they may be. After awhile, some of them realize they can pretty much get away with murder and will just be given a pat on the head, and not even so much as a slap on the wrist, and they start to abuse it. Not all have this happen, but it's more than possible to occur.
Also, while simple thoughts can often see through simple answers, this's a condition that's well documented in children being capable of seeing the obvious, whereas an adult has had their lifetime spent being conditioned to overly complexify things. For referance, do the "petals around the rose" game sometime, and yeu'll see whot I mean.
As a whole though, I disagree at this being particularly beneficial in most situations. However, there are certain types of mental retardation which tend to cause leaps in intelligence in other forms; autism often leads to notably higher raw intelligence, but lower social skills, the best guess I have on the matter is that it's an overspecialization of the brain, trying too hard to focus all resources on one particular type of thought process, and sacrificing other required ones severely to attain such. Many other forms of mental retardation can be classified in similar ways; idiot savants are an excellent example of such.
Mental retardation caused by disease, or injury, however, do not gain these benefits. Those cases are pretty much just screwed once that happens. Generally the benefits associated with such only occur from natural genetic occurrence, and most likely is just natural variation in genes in the typical "shotgun" style test each generation gets... the majority goes to the tried and true center, with some minor divergence, and a few rare cases get really far off the mark attempts just in case there's possibly something better out there. Evolution has to come from somewheres... some of these traits don't carry on, because they kind of suck. Others tend to be beneficial more than harmful, and find their way to the general populace. Unfortunately to do random genetic spread such as that, there's going to be a few people who pull the short straw when trying to find a straw that's longer than average...