I agree with you, no IFA I've offered one to without them asking has taken it. But I have had two players take the big league contract.
First one was an IFA who actually demanded a big league contract. No idea how often those come along; I've never seen another one.
Second one was a regular minor league free agent, an unsigned draftee from the previous season. Great hitter so I offered big league.
So I guess the option is there, but maybe it only works under very specific conditions.