How important is gear service?....Very.
CORROSION CAUSES DAMAGE
Salt water can wreak havoc on scuba gear. Even that freshwater rinse in the communal rinse bucket leaves much to be desired. Corrosion eats away at the smooth chrome surfaces, and eventually at the metal underneath, leading to the need for additional replacement parts. Rinse your gear thoroughly and let it dry completely before packing it away.
some of the icky stuff we see
BE SURE YOU go to an authorized repair center
There are SO MANY parts! There are parts that get replaced with every service (most of the o-rings, the seats, various washers), and there are parts that get cleaned and reused, like the main body, the springs, and pistons, for example. You do not want just anybody trying to put this puzzle together. Scubapro requires that technicians come to a repair clinic every two years to stay current, and only use authorized parts. If you come across a shop that offers Scubapro service in-house but is not a Scubapro dealer, beware! They are not backed by the manufacturer and may not be using the proper parts in your regulator! Scary.
frequency of service
Manufacturers used to generally recommend service once a year. Since about 2012 or so, most of them have moved to once every two years or 100 dives, whichever comes first. In my experience servicing gear, I would agree that a regulator, even if used only a couple of times in two years, should be serviced. Things start to get sticky and dry, and when that happens performance and reliability suffer. And you don't want to be worrying about regulator performance at 100ft!
things just wear out sometimes
Don't be dismayed if you use your gear so much that major parts need to be replaced sometimes. It means you've been diving!! This poor drysuit was need of a new main zipper...it happens. Parts wear out. That's the price we pay sometimes for having fun!