Updated information was added to the 1939 Register for many years after it was taken because it was the central register first for National Registration purposes, and then for the NHS. So adding a person's new name was not pointless; it was never intended to be a static document like a census, it was created to be a live register with up-to-date name and address details (and more) of everyone in England and Wales, and this is the function it fulfilled for over 60 years until the paper-based system was replaced with an electronic database.
Registering everyone where they happened to be on 29 September 1939 was just a way of capturing the whole population in one go and issuing everyone with an Identity Card. It didn't matter in the slightest whether someone was at their permanent address or was just visiting, because their address would also be updated whenever they moved, whether it was the next day, or decades later.
It is not a simple as that in 1939 it was designed as you say to be updated if required but the war was never envisaged to last as long as it did.
A little while after the war all work on the National Register was supposed to have been stopped. Questions were even asked in Parliament if it had been finished with and assurances were given in Parliament that the National Register was no longer being used.
The 1939 National Registration Act itself was repealed on 22 May 1952.
It seems though that those assurances were basically deliberate lies to deceive MPs, I say deliberate lies as the questions were asked on a number of occasions and the answer was always the same only the number ing systen was still required.
One example of an exchange is at
http://hansard.millbanksystems.com/commons/1953/may/21/national-registration-numbers#S5CV0515P0_19530521_HOC_51National Registration Numbers
HC Deb 21 May 1953 vol 515 cc2233-4 2233
§ 11. Lieut.-Colonel Lipton
asked the Minister of Health for what purposes national registration numbers are still required; and when the use of these numbers will be abandoned.
§ Mr. Iain Macleod
National registration has been wholly abandoned. Some numbering system, however, is necessary for purposes of the National Health Service and, for reasons of economy, this is based upon the old numbers.
§ Lieut.-Colonel Lipton
How is it possible for the Minister to say that national registration has been completely abandoned when people are required to keep, remember and make use of their national registration numbers? Is he not, in effect, still attempting to hoax the public into believing that national registration has been abandoned, whereas unless the individual remembers his national registration number he can find himself in all kinds of difficulties?
2234
§ Mr. Macleod
Oh, no. Any large scheme—for example, the National Health Insurance scheme before the Health Service was introduced—is bound to be based upon a system of numbers. We have used the same system of numbers, but national registration, happily, does not exist any more, and as a consequence a lot of staff and a great deal of money have been saved.
§ Lieut.-Colonel Lipton
We have to have a number?
Mr. Iain Macleod was the MP for Enfield West and Minister of Health (Conservative)
Lieut.-Colonel Lipton was the MP for Brixton (Conservative)
Cheers
Guy