I was excited when I first read that you could apply poison to a weapon, but quickly disappointed when I learned it was only for one hit. I wish it was set up to have one application work for say 20 shots for example, or 20 minutes, but one having it wear off after only one shot is honestly not worth the hassle & cost, unless it's super effective/strong.
They did this for balance reasons. I can't find the thread right now, but a few days ago someone worked out the exact damage potential numbers and discovered poisons that can do 2400 HP worth of damage are possible. That is enough to one shot anything in the game. BOOM! Instant death.
And as a thief and assassin, even the normal low level poisons are awesome. Pouring poison down a sleeping NPCs throat is the easiest way to kill someone silently there is. Or what about a group of enemies too strong for you? Hit one of them with a Frenzy poison and watch them turn on themselves.
Poisons are extremely useful since they can paralyze, slow, make enemies weaker to your attacks, sap their ability to do power attacks or cast magic, etc. etc. You just have to put in the effort with potions and poisons and get rewards out of it.