I'm saying players should not swing 3-0 regardless. I'm saying that the likelihood of a ball is greater than the likelihood of a cookie down the middle.
Not swinging 3-0, the best outcome is "free base, no out". and the worst outcome is "still in a hitter's count". The positive outcome is "34.5% chance of a hit". and the negative outcomes are "swings and the count is 3-1" or "generates 1 or more outs."
3-0 a pitch is called a ball 33% of the time (the number of pitches out of the zone is higher, as swinging at a pitch at your eyeballs counts as a strike there.) And if you put the ball in play (certainly not a sure thing) there's a 34.5% chance that it is a hit. 59% of 3-0 counts end in a walk regardless.
Taking a 3-0 pitch is more likely to have a positive outcome than swinging at one.
Edit: the positive outcome of swinging is actually 34.5% chance that, if you put the ball in play, it will result in a hit. You are, obviously, not guaranteed to put the ball in play on your swing.
Minor, but important, clarification.