There is plenty of maintinence on A/c and furnaces to be done. I would call another company that wants the work.
Humidifiers get nasty, pans in attics get filled with debrie and the traps clog up, or the pan leaks and you get sheetrock dammage on the ceiling.
Ductwork comes loose, filters need changing and things get dirty.
Heat exchangers go bad and you don't know unless the unit is inspected. Some units are hooked up to well water and the fittings start to corrode.
There are plenty of other things to look at, and sure you can wait for it to become an issue, but when it's 10 below outside...and you wake up and the house is 50 degrees..well..your doing the right thing.