Testimonials and case studies are a common way to get a second-voice onto the website to help sell your product / service from business-to-business. However, useful case studies are much harder to put together than a quick testimonial.
For a testimonial you can just phone a client and ask "can you give me a one-liner I can use on our website to say you were happy with our service?"
For a case study you (usually) have to get more clearance from the client to include specific information on the site (not everyone wants the world to know that they had to contract out development of their website / application / call centre...) and then you have to write it in a way that is of benefit and interest to people reading it.
I am curious; is the extra effort required to put together decent case studies wasted when simple testimonials provide the same effect - i.e. they reassure potential clients that you are trustworthy and can do what you claim, or do prospective clients actually respond better to detailed case studies than to generic testimonials?