I've always been curious what they would say, if I offered to pay $100 to $200 yearly to be on their preferred customer service list/no after hour charge, but didn't want them to come do the inspections. I don't have too much issue with what they charge to come do a maintenance call, but the real reason they have those programs is to upsell parts and repairs (which are sometimes needed, but its hard to know for the average consumer).
Its strictly anecdotal, but with the 4 houses I've owned I had more issues with my HVAC systems when I was on a maintenance plan than when I just did routine maintenance myself and didn't worry too much (how I started as a home owner and what I've done the last 5 years or so).