"So why SFRs must be named as different Memory Area? They are just a subclass of the internal RAM"

The databook for the derivative I'm working on tells me that it has 256 bytes of internal RAM. By your reckoning it would have 384 bytes.

"By the way, I cannot understand the logic by which you recognize different memory areas."

Oleg, draw a picture of the memory map of the 8052. I tried to do it with ASCII art but gave up. You will notice a 256 byte chunk of RAM, a 64k external address space, a 64k code space and a block of SFRs. That's all - four physically separate address spaces.

"And the main question: what all these Memory Areas definitions are for?"

It's difficult to talk about things unless they have a name. Or have I misunderstood your question?

"I see this task total useless; at least if we drop Addressing Modes criteria."

The whole point is that addressing modes are irrelevant - different ways of addressing something doesn't make it a different thing. An apple is still an apple whether I hit it with a hammer or a golf club.