For me, it was the simple situation---I knew that both knees needed to be done and I wanted to regain my life ASAP. I asked my OS and he said that I was in good enough shape and that he would do it. I wanted to regain my active life as soon as I could. My thinking was, "If I am going to have both knees done, why not have them replaced at the same time.?"
Looking back, it was the right choice, but, as Jamie pointed out, it does have its drawbacks. Recovery was longer for me ---and I went back to school too soon, so that slowed it down even more.
I had one surgical event, one recovery, one hospital stay, and no waiting for one to heal and to progress so that I could get the other one done.
Challenging? Most definitely. Frustrating? For quite some time. Worth it? Without a doubt.