The other benefit of double rings on Gottlieb drop targets, beyond protecting the drop targets themselves, is to protect the playfield plastics. A weak backing to the drop target will allow the ball to carom upwards and break playfield plastics. The upper drop target bank on El Dorado, the upper banks on Countdown, and the single target bank on Jacks Open are all prime examples where double rings will benefit the game and playfield plastics.
Basically any game that has a direct straight on shot to the drop targets is a good candidate for this. Glancing shots like the side DT banks on Countdown are not as succeptible.