Certainly it used to be a part of natural speech. Did you think it was an invented word? It’s just that that was a long time ago.
A corollary to the original question:
Is it:
That’s just who he is.
Or
That’s just whom he is.
:dubious:
Despite my comment, I do use “whom” in casual speech, too. But it depends with whom I’m speaking. 
“Who”
Why not whom? :dubious:
Because it is being used in the nominative. Why the dubious look?
A. Descriptively, because nobody says it that way.
B. Grammatically, because there is no transitive verb nor any preposition in that sentence for “whom” to be the object of.
I thought “whom” would be the object of “is.”
“IS” is not a transitive verb, and does not take an object. It does sometimes take the accusitive in colloquial English (“Who is it? It’s me!”) so it’s not completely illogical to think “It’s whom?” would be okay. “That’s just who he is,” though, has its own answer, because otherwise you’d say “that’s just whom him is,” and “him is” is both prescriptively and descriptively wrong.
Ignorance annihilated.
That form would be called a disjunctive pronoun. “To be” is a linking verb, so when you say “A is B,” both A and B should be in the subjective (nominative) case, at least in formal grammar. Now, what is logical and formal doesn’t always jibe with how people speak. To my ears, “it is I” as an answer to “Who’s there?” sounds awfully stilted, although it is formally correct.
I disagree, based on the fact that in my dialect (Californian) it’s perfectly grammatical to say, “Who is it? It’s me. Me and my friend Bob are here.” The first “me” might be a disjunctive pronoun, but it’s pretty clear that the second isn’t.* In colloquial English, the accusative form can act as a nominative in several (but by no means all) constructions. I could also say “It’s me that brought the cake,” but in writing I would construct “It is I who brought the cake.”
*Though I can see how it might have originated as such, and I can see that the Wiki article claims that second construction is disjunctive. Faugh, I say.
I’m not sure what we disagree about.
I’m not either. I think I only succeeded in confusing myself.
Thought I’d drop in, just to share my new favorite joke:
Knock knock.
Who’s there?
To.
To who?
No, to whom.
Damn! Stole my joke (which I think I got from here at the Dope).
Love it though. Sadly, only about 40% of the people I tell it to even get it, let alone think it’s funny. I guess that’s decent evidence that whom is really dying/dead.
I’ll write that one down.