I find them useful ;)
When you're in the Meadows, it may damage and kill passing fauna (free meat) as well as Greydwarves.
As you will know if you have not gone to bed, you will also get Greydwarf patrols coming out of the Black Forest. The stakes may not stop a Brute or a Shaman, but it will pre-occupy them rather than focusing on your other structures.
So, in the Meadows, Black Forest and even the Plains - 5 points of damage may be useless against Fulings, or Lox but will kill a deathsquito ;)
I haven't found them as much use in Swamp or Mountains though.