Should a critique always be positive?

I was listening to the Radiant Vista podcasts. I have only just found them so I have a year's worth to catch up on. They were discussing how people could send in pieces for critique and that the reviewers would always be positive. Of course nobody is going to enjoy a roasting but the whole point is to have problems pointed out so you can get rid of any weaknesses. IMO, so long as all points are fair, there is no harm done and I myself would rather find out my mistakes sooner rather than later.