I dunno, I nonrev at least 20-30 segments every year (international and domestic) and have only been bumped a handful of times. Just develop a smart strategy and use it! Airline employee's schedules can be awesomely flexible for this sort of thing. My rules are:

Fly mid-week
Fly off-season and avoid holidays
Fly to destinations served by mulitple airlines
Give yourself 'buffer days' (i.e. schedule a few days off after your ideal return date in case you are late getting back)

As for the exact benefits, they can be complex and vary widely between airlines. Most people don't use their bennies enough out of fear or ignorance, but for me it's a hobby/lifestyle that I enjoy. I think contacting HR is really the best resource for a general overview of the program.

I am quite certain that someone could get a lot out of non-rev travel. Your set of rules are a great "insurance policy".