Unless the souls in the soul coin can be freed, I think using their soul to move forward is still the better option than just leaving them to "waste", granted I haven't used them either. But anyways I think your PoV is wrong.
If Larian is going to change the lore I won't argue but in the existing lore using a soul coin in an infernal engine doesn't allow it to move on - it destroys it and the soul experience terrible agony and dies screaming. And we know that one of the coins we get from the moonrise bugbear is that of an innocent child.
