A client of mine was once convinced that despite my advice, nothing short of forensic recovery would put his old data at risk when he decided to donate some old workstations to a youth hostel. He wanted me to just delete the partitions the OS was sitting on, format and reinstall Windows. I did that on one machine right in front of him. Then I asked him to give me a few hours with that box on my own.
I returned the machine to him several hours later, with PDF copies of a few of "his" clients Tax Returns (complete with Tax File numbers) and a variety of other highly sensitive data sitting on the computer's desktop ready for the reading on a freshly installed copy of Windows 7 Pro. The entire exercise took about 4 - 5 hours, less than 30 minutes of actual hands-on work on my part. He was so grateful for my taking the time to show him what he was risking that I scored a $200 voucher to a high-end restaurant in Melbourne on top of my fee. Some people just need to physically "see" the proof of the pudding in order to believe.