Think about it. You are paying the company to come check your system and see if it's working okay. If they do find something it will usually cost extra to repair, just like when you get an oil change at an auto shop and they suggest fixing other things, etc. Change your filters yourself and call the company when something really isn't working right. I'm sure it's good to have maintenance agreements if you have a rental property, but if it's your home you can do without it.
We had one at our office for a period of time and finally said, every time they come there's some small thing wrong they want to fix for an additional cost. But the system wasn't failing. We did away with the contract.