You bring up a great point here. Anyone that has a PC on a warranty isn't going to take it to someone that isn't certified to fix it. Once they would open the case up their warranty would be voided unless their certified.
Numerous people have told me to start up my own shop around here but they miss some things. You have to have parts to test all the components. That means you need processors with different socket types, different style memory (DDR,DDR2,DDR3 as well as different speeds etc. If you don't have the proper parts to diagnose a computer what are you going to do besides guess?
I remember when my Grandmothers Dell Laptop was acting up and shutting off. I looked it over and could tell the exhaust fan quit working. I could have fixed it myself but their was no point. It still had warranty time on it and if I opened it then the warranty would of been voided. So I called Dell for her and sent it back. Kind of dumb really over something simple but its covered if something major happens to it down the road.