Grammatical Case (was: that long subject) War of Roses/Holmes?Figg/Walpurga

[Catlady (Rita Prince Winston)]

Geoff Bannister wrote in
<http://groups.yahoo.com/group/HPFGU-OTChatter/message/36958>:

> It's interesting that in several languages, the verb "to be" takes a 
> nominative and yet this "it's me" crops up. In French, which, IIRC, 
> had no case structure it's "c'est moi".  

I hope someone can explain this in a way that I can understand,
because my friend Lee only explains it in ways that I don't
understand. To me, the cases of English pronouns work like in this
simple sentence:

[Subject is subjective] [verb] [object is objective].

Exempli gratia: It bit me! I admired him. She defeated you. Thou wast
deceived by them.

"To be" is a verb. The word before "is" is the subject. The word after
"is" is the object. So why is there this weird unnatural rule that the
object of "is" should be in the subjective case?

I believe that the valid rules of English grammar are the ones that
evolved from centuries of usage by natural English speakers, blessed
with a non-elite language that wasn't taught in school and therefore
didn't have intellectuals inventing new rules or grammar snobs trying
to enforce old ones ("Say 'give it him', not this vulgar ignorant
slang 'give it to him'!"). 

Is this weird rule about 'is' one of the valid-to-me rules, or was it
invented by someone? If it is one of them, what were the old-time
speakers *thinking" when they so spoke?