Compared to the XBox Live services of the past, Playstation Plus is more than just "cost of admission" to play online multiplayer games. Many games/add-ons have discounts for being a Plus member, including some games being completely free.
So, let's say you buy a full year of PS+ ($50 bucks) and find a bunch of games you want for free or heavily discounted for PS+ users - you've made back the cost of the year's subscription... AND you still get that benefit of playing online multiplayer games.
Now, I should mention that not all games require a PS+ account for their online multiplayer. Games that are ONLY online multiplayer (MMOs) tend not to require PS+... usually it's up to the company behind the game in that instance. Make sure you check beforehand.