The PS + offer is on the PlayStation Vita (logically) to another so you can enjoy the on something like Rayman Origins, Gravity Rush or Unit. 13
The online store for example is useful for cross-platform titles like the Metal Gear Solid HD Collection, play on the PS3 and the Vita (UMTS variant assuming) to play in the car or train, or if the PS3 drive crashes one has his memory data yet.
The price is about 4-5 month quite aktzeptabel and definitely worth the Seim (unlike Xbox Live * cough *)
From my very clear recommendation