If you suffer no ill effects, then stay the course.
It's just a theory, but there may be additives in the higher grades that help with keeping injectors clean, reducing carbon build-up, etc., etc. Then again, it could just be "snake oil". Higher octane fules have a hotter "flash point", which inhibits detonation in high-compression engines, or lower-compression supercharged and turbocharged engines.
As your car gets older, some problems that might crop up could be solved simply by upgrading the octane you currently use.
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car