Even if you never fly Southwest, you can still join Rapid Rewards and enjoy the benefits of the membership. Its simply a frequent flier club, just like American Airlines AAdvantage, or Delta SkyMiles, United Mileage Plus, Northwest WorldPerks, USAir Dividend Miles, Continental OnePass, etc.
Right now Rapid Rewards members get discounts at SeaWorld, Busch Gardens, Space Center Houston, Hertz and others. Promotions and discounts change from time to time but many times you can get discounts with Southwest's other rental car partners (Alamo, Budget and Dollar) and their hotel partners (Hilton, Hyatt, Marriott, LaQuinta, and Choice Hotels).
Theoretically, you could earn a free ticket on Southwest in a year without even stepping foot on one of their planes, and without using their Visa card, by doing business with their partners above and others listed on Southwest's website. I don't think that you could get the SeaWorld discount with just the Southwest Visa card, I think you need to belong to Rapid Rewards. (I'm sure they assume that if you have a Southwest Visa card, you're going to be a member of Rapid Rewards anyway since you want to earn the points for your credit card purchases.)