Who's the Bell Tolling For?

When do you use who/whom?

I can't believe there's still a distinction between who and whom, because no one uses it, except in writing. Even a grammar maven like me feels it falls flat in conversational use unless it's rubbing up against a disjunctive preposition. But, here it is:

"Who" is always a subject, the agent that acts in a sentence or clause. "Who" is a pronoun that takes the place of he or she.

Who said that? He did; the guy with the big mouth said that.

"Whom" is an object, the person or thing that's acted upon. "Whom" is a pronoun that takes the place of him or her.

To whom did he say that? He said it to her.

PS: Can you imagine John Donne or Hemingway having written "Who the Bell Tolls For" instead of "For Whom the Bell Tolls"?