I think it's not a bad idea for single player games you play through and then sell. Watch Dogs would be great... $15 for a month, beat it, never think of it again. It's a better deal than buying it new and then selling it to Gamestop a month later.
Then again, if Sony would just make it a subscription-based service like Netflix, they will OWN this gen.
I agree if they do them for less than what it is to buy the game and trade it in it will be a good idea, i traded in watchdogs and infamous second son after i completed them both in a week..we probably won't end up with top titles like the ps3 had on ps+ because of this playstation now!