Currently each flashing RDHI (Red Devil Horn Insert) initially adds about 27K to your score and when all 3 are completed it adds about 120K to your score. Then when you start the cycle again (without draining the ball), they are each worth a little more individually and the completion of all three is worth a little bit more as well (125K vs 120k the 1st time they are completed for example). Basically, if this is all they currently do, they are virtually worthless with respect to a scoring strategy.

When a flashing RDHI is hit it also might currently (v1.65):
1) add time to the lower playfield on non-pros
2) and/or add to the time it takes for the playfield multiplier to timeout
3) and/or act as a primary shot bypass: after the primary shot has been hit, a non-primary white note has been hit to add to the song bonus. Normally, if that same non-primary shot then starts to flash because of a combo (you haven't renewed it with another primary shot hit yet though) you would not be able to add to the song bonus by hitting it (it would just score a combo or loop value), but if the RDHI is also flashing it will add to the song bonus anyway even without a primary shot renewal (only applies to the orbits and bell shot obviously).
4) Increases growth rate of song bonus by the aggregate total of RDHIs shot (including the bonus for completing three of them)
5) Increases base value of song bonus/jackpot (just as switching songs does)

Sealclubber, the way to add a ball during multiball was posted previously by another pinsider. In summary, after you shoot the primary shot for that song, the white notes are lit on the other shots. If whatever non-primary shots that remain are completed (or spotted with the VIP pass(es) ) during multiball, a ball will be added. So in SMASSA's example, you might have completed all but one of the non-primary shots during single ball play, and then in multiball if you happen to hit or spot the last non-primary shot you needed, a ball will be added if less than four balls are currently in play.

Although you can complete the non-primary shots you need during multiball to add a ball if less than four balls are in play, you might want to do some prep work during single ball play first if it starts getting expensive to start multiball: For example, maybe you have qualified super jackpot for Tour mb, but now need fifteen orbit shots to light the right ramp to start Tour multiball, because you have started it 3 times previously. Well, since it starts out as a two ball multiball, you might want to hit the primary shot and all but one non-primary shot before starting the Tour multiball to maximize the length of time you spend in the hard-to-qualify Tour multiball (make sure the last non-primary shot you need is not the right ramp when you shoot it to start your multiball).

I guess I veered off course with respect to the original subject. I think it would be cool, if in a future software revision, that completing the 3 flashing RDHIs spots a stand-up target jukebox song (TNT, Let There Be Rock, Back in Black) or at least one stand-up/drop target of one of the three banks assuming the completion of the RDHIs doesn't do something already.

I added a couple more conjectures to the initial post.