this is also stupid

Continuing with the theme "It's Stupid To Get Uppity About Grammar" -- and believe you me, that's a deeeeeep well -- I offer the following:

Sometimes there are people (even some I know and love) who, in response to "May I speak to Mary, please", think it makes sense to say "This is she." The thinking, I suppose, is that "This is her" -- which I guess we tend to say because we usually find the object form of the pronoun after a verb -- is incorrect since technically we should be using the subject form of the pronoun.
NOTE: if none of the above made sense to you, congratulations. You have paid the right amount of attention to grammar, and God shall smile upon you.
But I digress. My point is that "This is she" really makes no more sense than "This is her." "She" is not the subject of the sentence as if we're saying "She is this" -- we're saying "[The person answering the phone] is [the person you asked for]."
And when we wish to make the point that two people or names or positions or roles or whatever represent the same person, we say things like:

• That's her.
• I am actually him.
• He acted as me.

We don't say:

• That's she.
• I am actually he (unless we're utterly pretentious and precious.)
• He acted as I (ditto).

So I submit there's a general grammar rule (which you may call irregular if it makes you feel better) that after a verb we use the traditional object form of the pronoun. That's it. There, now you have a rule, enabling you to say "Well, okay then..." and go back to your room and dream up other ways to grind the spirit out of young people and make yourself feel informed and superior.