"Ratchet & Clank Future: A Crack in Time will probably be Insomniac's last 60fps game," writes the studio's Mike Acton, who examined improved framerate and concluded that it does little to drive purchases or good reviews.
Acton says Insomniac's community team took a look at the reviews of 47 top-notch games and found that while "there was a clear correlation between graphics scores in reviews and the final scores," there's no correlation between framerate and the graphics scores. In other words, good graphics make the game feel more fun to play, but whether it's 30 fps or 60 fps doesn't matter.
Take a look at his entire post; Acton gets into the studio's philosophy of what is and what isn't important, visually. And it's clear that 60 fps, despite the fact "one of the long-standing sacred cows here at Insomniac is framerate," is not going to be the holy grail of their development. In fact, it'll probably be 30 fps from now on.
How Much Does Framerate Matter? [Insomniac Games Blog]