I can't think of one good reason why these can't be toggled on/off, but I can think of plenty of reasons why they should. Going back and finishing off achievements has been a nightmare with quests requiring you to "DPS soandso down to X% then do Y". This one in particular was extremely annoying: https://everquest.allakhazam.com/db/quest.html?quest=7534, requiring you to DPS a mob to 10% (finishing blow triggers at 10.5%). Yes, you can un-equip daggers as a Rogue to prevent Assassination ripostes from deleting mobs, but that still doesn't negate Banestrikes and Finishing Blow. It isn't always advantageous to hit mobs for over 4 million damage.