Unless you buy every game as it comes out, PS+ is a great way to build your library. Remember, even after they remove the games, you keep them in your PS+ account as long as you're a member. In addition, all the discounts really make it worth your while. Many PSN games are discounted 33% on day one. Hell, they gave us Double Dragon Neon free on launch day...
I've received well over $250 in free games and discounts over the past year (and I'm talking market price, not retail MSRP). If you're an avid gamer, it's worth it.
Don't you only retain access to your games as long as you remain a member? If you drop your membership subscription, don't you lose access to all your amassed games, even if you resub later?