here's the rather unpleasant truth with the 30 day requirement: it is to weed out stupid people who can't read/follow simple instructions.
otherwise you are right, there is no real purpose to the requirement. however, there's no real legitimate reason why you didn't register either, since it doesn't cost you any real effort nor do you have to go overly out of your way.
To preface this, I know they put the serial numbers on the video card box, but I never think about registering at that point. Building a new PC is like a nerd's own personal Christmas... he does not worry about silly boxes.
I've actually been pretty good about registering my video cards as of late. What would happen to me in the past is that I'd remember to register the card mid-build (while putting it in the machine). The only problem is that I typically cannibalized at least something from my old machine (like the case) that required me to take it apart. So I wouldn't really have a computer unless I went and got my laptop.
Now I just build entirely new PCs each time. When I upgraded to an i5-2500k from an i7-860, I could have just swapped the motherboard and CPU, but I decided to just build an entirely new PC. It's more expensive (obviously), but it does provide a nice benefit of a
quick and easy spare in case something strange is happening to your PC and you've got a raid in half an hour.
But anyway! The benefit is that now I have a PC still available to use mid-build and I just register when I get to the video card.
Although, someone else mentioned rebate windows. Rebate windows are a lot different in nature and I don't think really apply. Rebates typically include a set amount of funds that are set aside to pay for said rebates, and you don't need money tied up for years waiting for people to finally send in the forms.
Of course I'm not placing all of the blame on eVGA. Technically I screwed up by not registering it soon enough. That doesn't mean I have to appreciate their policies though, and last thing I need is to spend $650 and get burnt again (even though I doubt I'll ever spend that much on a video card again).