Why should I spay my dog?

I have a 2 year old female chihuahua, Fergie (whom I've had since 4 months), and a fixed 7 month old male pom chi, Drake. They are both strictly indoors, but I take them out to potty. What health concerns should I worry about if I choose not to spay her? I'm not worried about her getting pregnant because as I said, she is strictly indoors, never around other dogs, and never out of my sight. I need serious answers. I had an appointment this morning and backed out because I felt bad for her. After neutering my baby I don't know if I could go through that worry again! There's always that chance they won't wake up after surgery. Thanks in advance for your help.