They give 10% science and the ability to build missionaries. They also crap out at Scientific Method I believe. Obviously whenever I have the wonder for a religion I have a monastery and a city pumping out missionaries to get my 1 gpt. However, I rarely build them otherwise; sometimes, not even in my science cities. I just find I usually have better things to build and I don't like the fact that they are useless before long. What about everyone else? Am I missing out on a great building?