"For example, if I am looking for Ruby on Rails experience and I have candidates with strong python and Django experience are they off the list?"

I think in most companies yes, they're off the list. Bottom line-ism is king these days. Companies see bringing someone in and up as a cost, not an investment. All they want to do is pull from the well, but not put anything in. They want perfect fits, and they want other companies to make them. It sounds like a frighteningly random and risky policy.