If you're truly healthy and under 50 you don't even need an annual physical. Even over 50, every two years probably suffices, and there is some debate over what constitutes "healthy" (as far as blood pressure, cholesterol) and what preventive measures are necessary (colonoscopy probably, PSA test almost definitely not. There are now alternatives to colonosopy that are sufficiently diagnostic for lower-risk people, but then your medical provider doesn't make money off the procedure).
Actual diseases and disorders are a different story. I suspect even with something as serious as diabetes, if it is being well-monitored and maintained, some people might be paying for more doctor's visits than they need (and the frequency and dangers of overtreatment are also pretty well-documented).