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)