I rescued a cat a fair few years ago now - 3 weeks later she had kittens - the rescue centre didnt tell us if they knew she was expecting and we just thought she was a tuby girl - never felt a thing until a week before she gave birth - I found them all good homes myself as the rescue centre didnt want to know. I also rescued my staffy girl about 7 years ago now - they asked all the right questions even had us taking our dog at the time up to spend some time together to make sure they would get on - its a great idea - we did that for just over a week and then Jen came home with us and is still going strong and is the boss of 3 Bordeaux
My old cat was rescued through Cats Protection and I had no probs with them.
I think some rescue centres are very well run but have heard stories of a few bad ones - I totally agree that most people want a "breed" of cat so go to a breeder not the rescue - My last cat is a British Shorthair and was purchased from a breeder - no problems with them either.