I rarely see them in SF and they're rarely in the problem areas anyways.

I've lived in SF my whole life, taken MUNI a lot (gave up on it mostly, I drive everywhere now) and I continue to take BART; I've been "fare inspected" twice.

I usually only see the fare inspectors when they've caught someone who then got belligerent and needed to be arrested.

It apparently didn't take long for this to happen at a station with new fare gates as well -> http://nbcbay.com/EVt6G4o

One of the top complaints I hear about BART/MUNI is about safety and if they can't even catch fare evaders, how are they ever going to find an actual criminal?

Growing up MUNI did have fare inspectors, albeit not too common and mostly on the light rail. However, I haven't seen a fare inspector in years, and fare evasion is noticeably worse these days.

There are inspectors, they always start out of Van Ness Station and ride out to the lines they are checking for the day, then back out Van Ness. Statistically, you are most likely to get checked at that station as a result.