You're missing the biggest point of onlive - they maintain the gaming machines for you. Instead of you having to upgrade your computer every few years, and having to live with a sub-par gaming machine towards the end of its effective life cycle, onlive's servers will be continually upgraded to keep up with the games.
So now you can game on your netbook, your Mac, your TV or even your cellphone.
There's inherent advantages to this approach - you don't have to worry about downloading/installing games, hardware specs are always properly matched, you can play the same game, pause and restart from different terminals, you can share replays with your friends, etc.
But one of the biggest things is that since all the gaming code runs at the server end, this will almost eliminate cheating. No more aimbots, wallhacks, network hacks, etc. It's all just mouseclicks and video.
You'll have to decide if $15/mo is worth it for you, but I think the charge is reasonable given what they have to support. You definitely get more for your money than a subscription to XBox Live.