Most dive equipment was not serviced yearly, perhaps every couple of years, but it was entirely up to you; there was no one around to require or browbeat you to do so. Often there were no repair depots anywhere near but the main reason was that dive gear was perceived as being reliable. Companies like USD bragged that their regulators had been in use for X years without one fatality traced to mechanical failure. Indeed, the ads were truthful, the two hose regs and other gear of the time were very reliable. Exceptions, like wetsuits, could be easily repaired by the user.
As diving gear became more complex, there were growing pains. Certain components such as the inflator valves on BC's had reputations for freezing up due to salt. The designs were improved. Back then, regulator high and low pressure hoses would fatigue at the joint. Stress relief boots saved the day. Almost all of the old problems are like that, history. Oh, there are still recalls and shop bulletins detailing various problems and if you have a piece of gear which is new to market, you will want to be aware of any such goings on. Avenues like this one can help, BTW.
One reason for the current stipulation of yearly service follows from the original motivation for yearly checkups by doctors, economic. Churn the business, bring customers in. This started with Scubapro and their marketing blitz(scuba schools) in the early "70's. It caught on.
Like the human body, regulators are basically reliable devices, same applies to most other diving stuff. However, neither take well to repeated adjustments and especially assembly and disassembly. In this vein, dive equipment is made of rather soft, fragile materials; brass, rubber, fabric, plastic. In spite of this, most of the gear as assembled by the FACTORY is quite durable assuming common sense maintainance. The various parts, even those considered "expendable" such as O rings, last a long time. However, whenever tools such as wrenches, spanners, dental picks, screwdrivers, etc are applied to these devices there is a small but detectable amount of damage. Should the carefully spec'ed parts be inadvertantly or intentionally substituted with something else, even a tiny error such as an O ring of different durometer, there is potential for trouble. A bit of dirt or a flake of chrome falling into a high pressure block goes a long way. There is also the potential of human error of a type leading to especially dramatic problems.
So, when do you overhaul or have your equipment "inspected" by a shop tech? That's up to you. If it's a vintage piece of gear with some known problem like the inflator valve, yearly, or better yet, replace it. For example, get a newer type with a booted, sealed valve and extend that service out to maybe 3 years. A modern regulator, 3 years. Valves, 5 years. At least, that's what I do. I work on much of my gear but will definitely take something which I'm not familiar with to a pro. Can you repair a computer? I can't.
It's up to you to decide how much grief your gear takes as a result of travel, handling, diving, heat, pollution, etc. It would take a lot to justify yearly checkups. If you feel your life could be in danger and the thought of unreliable "life support" gear makes you hyperventilate, do the yearly thing. Similarly, if your warranty requires it, do what you have to do.
