I am always playing yesteryear's games. I tend to upgrade to be able to play previous gen games. It gives me a far wider selection, patched games as mentioned before and I can simply take my time to finish a game in stead of having to rush because the next big thing is already alround the corner.
Other reasonings are of course the price: for example I waited five years to buy a PS2 simply because the console was by then affordable and there were already games being downpriced (and I needed a decent and cheap DVD player). Now I'm going on a rampage collecting all the good games that are at 1/3 or even 1/4 of the the price compared to when they were just released.