If PlayStation Plus let you keep those games permanently (instead of taking them away if you end your subscription), I might sign up.
For $50 a year, you're currently getting several hundred dollars' worth of content (MSRP) for free right up front, setting aside all the discounted and "free" games that come down the pipeline over the course of that year. You pay more than
that a year to get the
privilege of online play from Microsoft. Why in the world
should Sony just
give those games to you? You're already getting
far more than what you pay for the service.
One problem with Plus is that if there is a game you
do really like, once you download it as a Plus game you
can't purchase it permanently or receive it as a free gift via an in-game voucher (something I ran into when my copy of Starhawk had a voucher for a PSN download of the PS1 game Warhawk), at least not until you're no longer a Plus member.
My big problem with Plus, though, is that Sony has a nasty habit of giving certain games for "free" that everyone already bought anyway (like they just did by giving away The Walking Dead Episodes 1 & 2). I wish they'd spotlight more overlooked games that are not really selling anymore.