The most difficult alternative

Poison units are sometimes useful, especially when you need to quickly get to level 3 and 4. And in the endgame, they are twice as useful.

I agree, if you have the time to take them to the endgame, or if you don't need to go too far...
Also, if a level 3 unit won't be in the endgame, it's easier to get Venom units - you often don't have enough crystals for Vampires.