platforms in gemfile

Have any of you used a line like this in your gem file?

    gem 'therubyracer', :platforms => :ruby

I used it and it isn’t behaving the way the docs said it should. People on SO are giving me workarounds but I’m trying to figure out if the docs are wrong or I’m reading them wrong.

More details: http://stackoverflow.com/questions/8421321/does-using-platforms-in-your-gemfile-work