After super dude it'll reset to the bottom, iirc. I think I've seen people get all the way through the dude-o-meter in competition play before on an easy-ish-playing Dr. Dude.
It's a spare. They had the 3-ball trough on hand, and the cost of a pinball is low enough that having lost-ball protection for games on route was worth it. IIRC almost all the (Williams/Bally) multiball games post-whenever they designed the 3-ball trough have a third ball in 'em, even if the game only has two ball multiball. (I'm not sure if Stern/DE/Sega or Gottlieb did any two-ball games offhand.)
Great to see you're enjoying Pinside! Did you know Pinside is able to run thanks to donations from our visitors? Please donate to Pinside, support the site and get anext to your username to show for it! Donate to Pinside