@Abhishek -

I agree that this functionality is Plugin territory, but I won't complain if it does make it into core. I would find it occasionally useful.

@Scott R. Godin -

It is deliberate design intent. WordPress assumes, and intends, that the user will choose either the Visual editor, or the HTML editor, but not both during the same editing session. If you want to be able to switch between the two, it is simple enough to filter the TinyMCE init configuration to prevent the wonky behavior caused by the switch.