I agree, and yes, its been discussed.
Especially in areas like where labman, tkrussell, speedball reply... Many times their answers to technical questions stand alone, without agreement. I might agree with what speedball says, but who am I to "prop up" his answer? His expertise makes it stand without rating.
Whereas the posts where the issues are more emotional, like where I post most of the time, tend to get more feedback. My posts in the science section rarely get rated, but in the relationship sections do.
So I think that's part of the problem. When someone is clearly an expert in a field there are times when the rest of us feel funny about rating an answer that is clearly correct.