Quoted from frenchmarky:
When I'd shopped my Magnotron (forgot it wasn't the 2 player I had) I assumed there were just the posts around the captive ball with no large ring. And now that I think of it that makes the shot not worth as much since the ball won't bounce back and forth as much (or at all) and rack up more bonus. Probably hurt my overall opinion of it.
Think it's a timing thing, but bouncing back and forth (which does happen a lot) only scores 1x (3 bonus advances for 5-ball, 5 for 3-ball) - I was worried about a double score making the captive hole even more valuable, but I've never noticed that. As opposed to the upper kickers - where the ball does get hung up and score multiple times.
Still, it's pretty easy to hit the captive ball if one catches the ball, either shooting same side (left-left or right-right) or crossing over - so all it takes is 1-2 good balls to rack up a good score.
I do find the center captive hole is easier to hit with the big ring; when it was small post rings, it was indeed, as pinwiztom notes "posty" (love that term) and hard to get the ball to settle in. I sat down with the schematic not too long ago to figure out how the captive hole value worked - it's related to the number of bonus advances: 0-3 = 1000, 4-6 = 2000, 7-9 = 3000, 10-12 = 4000, 13-15 = 5000). Initially I thought it was related to number of times you hit the hole, and since the captive hole also advances the bonus, sometimes it increments the hole value by chance.