We recently had our 1998 HVAC system replaced. It was a reliable system that never had any issues. From what we have been told and read the newer more high efficiency units should be serviced regularly to keep them running at peak performance. Ours came with a three year 6 month maintenance plan. We will see how it goes.The old saying they don't build them like they used to comes to mind.
So what does that mean?
Correct pressures? Cleaning coils? Testing safety devices? General safety check?
A properly installed system should have steady pressure. It should not require adding refrigerant. If they are adding on a system under 10 years old, they screwed up the install and they should instead fix the leak.
Coils. The internal evaporator coil should never get fouled up if the install was good and you regularly change your filter with another high quality filter. Any dirt on the coils is a sign of a leak in the return, and you'll likely see the blow-by on outlet vents too. All around, really bad. Cleaning the coil is like liposuction. It helps, but doesn't get at the cause.
Outside, you can clean your condenser coils with a good jet from the hose. Should be sufficient. Keep debris out and cut away foliage.
On a properly running well installed system, most should by DYI. A sophisticated DYI can test some of the safety devices. But there shouldn't much worry about a cracked heat exchanger or any of that kind of thing for at least 7 years. OP mentioned it was a heat pump, so you don't even have that worry.
One caveat: If you have a gas appliance, and you feel better having it checked by a pro, by all means, get the contract. It is still a good idea to learn a bit about what a flu pipe looks like, what it does, and how it fits together. Always have a carbon monoxide detector if you have gas appliances. With a little bit of knowledge, even gas appliances can be monitored DYI, but I totally understand the desire for a 3rd party check.
The service isn't going to do anything to improve the performance of the reversing valve, or fan blower, start up capacitors, or any of that. The primary adjustment they can make is pressures. And if installed right, the pressure shouldn't change. They don't swap out start up capacitors as a matter of course. They wait until they fail, then enjoy the call they get from you because you are on contract.