I don't have first hand experience with EVGA warranty service, but have seen they are good at standing behind their products. My opinion on extended warranties in general is that they are a big profit center for companies that offer them and therefore not a good deal from a consumer standpoint. I think with VGA's this even more so, given will you even want the card after the initial 3 year warranty expires?
You have a point, but equally if they did break, you could get it replaced with new then flog it....
That could be one benefit, if you're still using the card after three years. Also the fans will almost certainly go out before anything else and way before the warranty ends if using it 24/7 for mining. To maximize the return it would be better to sell it off before then.