It's impractical to always get a 20 hour knock-out, and I had assassins ask me to KO people for them for that long. Not only did I not know how but I wasn't about to pass up my own loot to some cheap tactic that needed to be used on the dude asking me in the first place.

In any case, while you can make some really strong poisons it's a question of whether you can keep your opponent from simply getting a return potion from his sack and quaffing making your 48 hour neurological poison moot.

Plus it won't make the special affects on some poisons fire more often (forget on mind control, spellcasting negation and movement confusion with neuro)