You know, they say to never get a second cat for your first cat. You should get a second cat for yourself, and hope it becomes friends for the first cat.
I can say this is definately true. I got Thomas because I wanted a second cat, not for Kitty. Which is good, because they don't like each other much. But if I had gotten Thomas for Kitty, and not for me, I would probably have gotten rid of him by now.
Also, I had Kitty as the only cat for 6 years before I got Thomas. We lived in apartments, Kitty and I. And I worked long hours. She never seemed to care as long as I fed her, brushed her in the morning before I went to work, and slept with her by my side every night. She spent her days napping and playing by herself and never really seemed lonely, just alone.
I definately think Thomas has enlivened her life, but not necessarily in a good way.
So, a second cat might be good for your cat. But your cat will be fine without a playmate. I'd only get a second kitty if you really want one and are ready for one.