There are those of us who run outsourcing companies that value the clients far more than the clients trade secrets. It would NOT be in the best interest of the outsourcing company to take the things they are doing for one client and use it for another client, simply because things like this always get back and when they do, you have lost not only one but TWO clients. Losing long term business isn't near worth it to me in my opinion. To anyone that would abuse this, they aren't serious about what they do and shouldn't be in business anyway, much less in a matter such as this.
This is how I think anything should be though, it should just be a given that there is a measure of confidentiality, whether it is for outsourcing companies, designers, mainstream, porn or any other business, it should be held at a standard that shows respect for BOTH parties involved.