I think the bigger question is why WOULDN'T you want to pay for PS+?
I've been using the service for about two years now, and it's worth every penny. The service practically pays for itself within the first few months. Seriously, you get free games every month, for 12 months, all for $50.
And these aren't just shitty shovelware titles, either. This month in the states, you get Deus Ex: Human Revolution, Saints Row 3, Machinarium, Uncharted 3, XCOM: Enemy Unknown, and LittleBigPlanet Karting.
In the past, we've had Warhammer 40k: Space Marine, InFamous 2, LittleBigPlanet 2, Just Cause 2, Darksiders, Spec Ops: The Line, The Cave, Vanquish, Bioshock 2, Borderlands, Demon's Souls, Resident Evil 5: Gold Edition, Sleeping Dogs, Plants vs. Zombies, and more. (Full list here: [link]http://www.reddit.com/r/PlayStationPlus/wiki/list/fullna[/link])
All for free.
And not only do you get free games, but you also get early access to betas, automatic updates (which is a godsend for PS3), game trials before you decide to buy a game, and 1 gigs worth of game saves at your disposal.
I can't wait to see what Sony does for PS+ for PS4. But I have a feeling there won't be that many free titles at launch.
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...That was probably more of an advertisement for PS+ than it was me praising it. But seriously, this shit is guuuuuud.