Its like the most useless skill on the skillpage. You need soulgems? Go farm a boss for awhile.
Occasionally you loot an empty soulgem but my stack of filled ones are just that much i dont even bother picking it up or use soul trap to fill it up.
Maybe if i just sold a stack of a 100 and my current stack of gems is low again and find an empty one, yea ill go find a mob slot soultrap and fill it, but other then that? meh, why bother.
Soul gems shouldnt be so easy to get, let alone filled ones. Imho, it should be removed from bossloot all togheter and instead get a 100% drop of an empty one according to your level in a treasure chest, put that soultrap skill to work.