Also, they do not do what professionals in the field could do if they so chose to and I would say that any true professional worth their job should be doing what you say critics are responsible for.
As for bias and defensiveness, I think you will find that many critics works take thier lead from the writers personal standpoint regardless of the attempts at objectivity. This is simply human nature.
Finally, you again bring in the "thumbs up/thumbs down" arguement. I don't think in such simplistics terms but I do think that everyone has the responsibilty to act as a critic in regards to everything of importance (professional or otherwise) that they partake in.
My final view point that I will end with is that critics should get on with producing something other than writing works that cover issues that should be considered by people working in that domain. People who do work in that domain should stop being halfarsed about their work and do a proper job of it.