PS+ offers something that can not be found anywhere else, and that is free games. You can argue that they are rented games but you get to keep them as long as you are still a PS+ subscriber.
I for one, don't really care about playing most games online, except for some of them. I hated the Idea that i have to pay to play online, but for me PS+ is a great service and it's not just there to play games online. I was one of the first people to subscribe to PS+ when they announced it and I'm still a subscriber and that won't change anytime soon.
Didnt MS announce they are giving away free games now with Live? And they now have dedicated servers? Sony are going to need to give a hell of alot of free games away if they dont have dedicated servers on the go while charging the same price as Live.