It's not useless when as was pointed out so many times that everyone likes to ignore, games can already load up past 3.5GB and don't have problems. Nvidia did not make adjustments for every game and engine in existence. Especially not some older ones I was messing with.
28GB/s is almost useless for such a fast GPU, maybe the games you see are only loading less important data on those 512MB (driver optimization working well) and that's why you don't see the difference, but at some point it could happen, because there is a massive drop in speed from the 3.5GB to the 512MB