Someone had said that even atheism has its place in Hinduism:
here's the Wiki entry about it
Finally, someone asked, simply, "Why unregister?" and someone else added to this argument, saying, "Good point. I don't remember explicitly registering, and given the sheer diversity of practices and beliefs, not to mention the lack of a single organizing authority, why the need for an explicit unregister process? One might as well just stop believing and go about one's life, no? If however, the unregister question is related to the legal aspects of belonging to a religion (personal laws et al) then I don't have an answer."
Well, it's interesting to me that the need to not belong to a specific group is often as imperative as the need to belong. How often do we hear people say, rather proudly, "...but I'm not like them!"
Some people obviously are not comfortable with an amorphous state of an unstated credo.
Some religions (though not Hinduism) have no place for those who do not believe; they are labelled "pagan" or "heathen" or "unbelievers" and can be persecuted and killed....the closest one comes to this in Hinduism, is the term "ajaata shatru", one who is the enemy of those not of one's caste. In fact, I am not sure if the 'jaata' referred to means caste at all.
However...I feel that the "registering" of a person as a Hindu may not be a single discrete act, but a process...the priests at every ceremony in one's life, invoking the Hindu gods, and the many "Hindu" practices one almost automatically follow, throw up the network of Hinduism around one. So the registering is still quite thorough, and for men/boys of three castes, the Upanayanam or thread ceremony was certainly a specific initiation into Hinduism...theone who dons the sacred thread is now a born-again (dwija) Hindu. That's a pretty intense initiation rite to me.
Someone I know registered a sworn affividavit that he was not a Hindu; this was the result of a confrontation (if I recollect right, with a school or college principal) that he had to take a specific stand on. Most of us don't take such dramatic steps, but just muddle along, each with our own version of Hinduism. I am still not sure what place there is in Hinduism for those who do not know whether to believe in a religion or not, and are content to live with that uncertainty.
I also do not know what the changes in one's legal status will be, if one "unregisters" from Hinduism...constituionally, since India is a secular nation, there should not be any change. If this is so, then, truly the need to "unregister" is only to set out, in black-and-white, one's beliefs...or lack of them.