While I agree that it's documented well enough that once you hit the situation and read the doc you should be able to figure out why without diving into the C,

While I agree that it's documented well enough that once you hit the situation and read the doc you should be able to figure out why without diving into the C, I think the documentation still fails in the sense that it doesn't give you any guidance as to why or when you would choose which one. That said, I really don't consider this magic as another subthread suggests. include? just has an optimization for numeric ranges, but otherwise behaves exactly as Enumerable#include? should.