It would be convenient for those who like to replay older games, but since we're talking about a minority, it doesn't make much sense for Sony to waste time and resources in such a feature. Let's not forget that creating a ps3 emulator for e relatively weak hw such as the ps4 is no small feat.

To be honest though, I don't even have the time to keep up with the endless stream of high quality games releasing on the ps4, to the point where I don't know if it would make sense for me to buy GOW at D1 since I would probably play it several months from now...I don't see why anyone would be willing to play ps3 games when there's so much goodness coming out on the ps4.