Because we get free games and while MS is starting with it too: They're offering rather old games (we're talking Gears 1 and Halo 3 here but still its a start.)
I was subscribing to PS+ before it was needed for PS4. I have like 20ish games in my backlog thanks to it. Its due to PS+ that I fell inlove with Sleeping Dogs! So yeah you cannot debunk the value you get there.