The annual requirement for regs (and other gear) was called out for two reasons: 1. It was recognized that there was a "down" time for most every one's diving, so the rationale was to service the gear when it was not in use or needed. 2. The other consideration, and the REAL reason, is the actual usage of the gear.
It is a lesser known fact that gear that is not used frequently is in more need of "servicing" than gear used quite frequently. The reason, The "seats" (fine edged surfaces that provide the positive seal against unwanted pressure changes) get very worn, if not exercised on a regular basis. That isn't to say that these seats do not get worn with use, just that they wear less. It is the accepted thinking of the day to calculate average use of a regulator at 100 times per Annual cycle. I can hear it already; "I dive way more than that, what should I do!" Hence the average use of the regulator. The regulator can go many more cycles than that without an incident, but in today's litigious society, an annual requirement is a safe time frame.
The one's that service their gear more often, could very well need to have the service done, due to the extreme use of the regulator. Others may do this to ensure the continued safe functioning of the regulator, and their peace of mind. Diving conditions may dictate the increased attention to one's gear. PSD divers come to mind, and those diving in contaminated water, as well. Another reason may be that the diver is acutely aware of the performance drop in the regulator, so wants servicing performed more frequently.
What one must realize is that annual regulator service is PRENTITIVE MAINTENANCE. This should always be the mentality when using and maintaining life support systems. You do not want to wait until it breaks to fix it, you want to be proactive and anticipate your maintenance needs!
I speak from a very experienced background, having worked on scuba regulators for over 15 years, but having maintained aircraft for 29 years. I do not mess with life support systems. My dive team has me maintain their gear. Who better? After all, their gear is my back-up.