I think I see where your coming from. but, maybe I misunderstood, some. Why would you want 'anyone' to work on one of the most important (and expensive) systems in your house? Why wouldn't you want a licensed tech certified to fix your equipment? Maybe he just got a service bulletin that explains your exact problem, for example.
I also think the 'bad guys' are very prevelant (at least in my area). Small cost cutting measures that will really hurt a system are easy.