when the search engines index the pages.
Then you have brand shift which is not under your control but is tied intimately.
Personally I have kept an eye on this phenomena, and this article has made me go and sort out a DNS server for my domains.
It is just a touch unethical on the registrar's behalf, but they really only sell you the registration - the name resolution has to be done using a name server, and that is not what you have bought, it is an added thing. So, yeah sure most would not buy a domain that could not be resolved, so they do tend to give this away to drive sales. But, it is not their responsibility on the purchase of the domain name, to provide a name server to handle the requests. That would be a different product.
It is like buying a car with no petrol in it. Most places will give you half a tank or so, until you can get to a petrol station. Of course what they do here, is say no need to buy petrol as long as we can advertises on the wheel caps etc.
Thing is most people don't understand DNS - and to be fair it is not that simple,.Until you realise that we don't need any of these governing bodies in DNS you have not understood DNS. The governing bodies are useful to have but not a necessity. If enough people wanted to drop the current DNS system - it could be done overnight, I am not sure why we don't do it actually, it is a good way to make an elitist tech only DNS system, IP 6.5 anyone? - keep the barbarians from the gates, without going too dark netesque.