At the rescue where I adopted both Jazz and Scout they really encourage people to take a cat home to try it out for a week or so before officially adopting, of course only if your application qualifies you. Anyway, their theory is it's the best way to make sure you are getting the right cat for your home and even if it doesn't work out at least the cat had a week or so out of the cage to get some exercise and a bit of a normal life. Some of their cats are confined to cages for months at a time so anything is a great break for them.

On that note I believe your fostering is a great break for them and so much better than the alternatives. Plus you are helping to socialize and increase their chances of adoption. I know a lot of people would like to know things about the cat before adopting, like behavior, temperment, how they are with other cats, etc. Fostering is really the only way to know these thing. I think you are helping them in so many ways.