Well, Defeatist does make Archen/Archeops kind of a lot worse, but they're not useless by any means. Just try and maybe switch out if you know you're gonna go under 50%, or you can even teach it roost.
It's kind of the same thing as Game Freak did with Slaking, Slaking would be Uber if it wasn't for its ability.
It's Game Freak's way to not make the Pokemon overpowered, I guess.